There can be numerous reasons behind slow internet...
Your internet slowing down can be due to numerous reasons, it could be a problem with your computer, , adding more devices, router, phone line issues (if you have ADSL), the ISP has slowed down, or the website your visiting is using more things.
Firstly lets try and eliminate anything that could be going on with your computer.
On your computer are you running any type of file sharing software in the background for example uTorrent, Limewire, Bearshare, etc if you are stop these. These can slow your internet down because whilst they are running in the background anything you have downloaded with them will be uploaded and shared with the community using your bandwidth, that's how P2P sharing works.
There are also other types of software out there that have P2P hidden in them and these can also slow your computer down - for example World of Warcraft uses P2P to install updates, some online players use P2P to share content with other users, some online TV players had Kontiki built in which shares files without telling you, BBC iPlayer used to use this (they've since stopped using this)
There is also other software that can be using your PC and slowing it down, some of it can be spyware, adware, malware. Sometimes even with the best and most up to date antivirus and antispyware in the world these can be missed. If you have any strange toolbars installed, like Alot, Ask, etc these can slow your computer down, try uninstalling all the toolbars (Google toolbar is normally safe, but even that takes up some internet time)
Then there is Adobe Flash, Flash can really slow your computer down, especially with more and more websites using Flash for interactive advertising down the side. You can disable flash in the plugins, and usually disabling it you will see a serious increase in speed, especially if your on a slow internet connection (I'm in a rural area of Shropshire, UK and we have a speed of 2mbps (if we're lucky), just turning Flash off sees a massive increase in speed). The only problem with turning flash off is that some websites need Flash switched on to work, so you need to turn it off when you don't want your computer to spend ages trying to download an interactive advert, and then turn it back on when you need to use websites like YouTube, or Farmville, etc. With HTML5, Flash should disappear and that should speed things up, but until more companies like YouTube adopt a HTML5 version of their websites over Flash we're stuck with this old technology.
Another thing you can try to speed your connection up is to switch to using Google Chrome, I know someone has already suggested you to try Firefox and you found no difference. With Google Chrome you should find an immense difference in speed, I switched years ago from IE to Firefox and then went to Chrome and now whenever I go on another computer that's running Firefox I find it so much more slower than using Chrome.
Now moving on to problems beyond your computer...
Another problem that could be slowing your internet down is the amount of devices being connected to it, and I'm not just on about in your house.
Unless you have an absolutely dedicated line all internet connections at some point share the connection with other people, this is the contention ratio. The higher the contention ratio is the slower your internet will be, and the way it works is for example
At your ISP's exchange they might be supplying internet in at 100mbps and then splitting that 100mbps between 50 houses in the area. Now if all 50 users were to go online all at the same time and use their network flat out then you would expect each person to be downloading at 2mbps if you split it fairly (100/50 = 2) however it's not as simple as that. Some people in the area can end up with slightly faster speeds, which is taking it from the others, there is a restriction on how much it can take so if no-one else in the area was using the internet at the time you wouldn't automatically receive the full 100mbps, the speed you'd receive would depend on numerous other issues as well, such as type of cable (fibre or copper), distance from the exchange, and any restrictions your ISP may have applied). But you might find that your getting slightly slower speeds because some other people in your area are getting slightly faster.
Of course with more and more people using the internet and more and more devices needing access this is also increasing demand. So years ago you might have had this 100mbps service 10 people who had the internet in your neighbourhood but the exchange had capacity to share that across 50 connections, then over the years 40 more people in your area decided they want the internet, suddenly you now have 50 people and they are all still sharing that 100mbps service, so now it's going slower because the exchange is working at full capacity. In addition to this years ago each household might have had one or two computers connected to the internet and only using it to surf the web and send and receive e-mails which doesn't use the full 2mbps. But now you don't just have one or two computers in each house, you might have one main desktop, a couple of laptops, smartphones, tablets, interactive TV's (and in some cases even interactive white goods like the fridge!) and all of these are competing for this speed, and many of them aren't just doing a simple browse the net and download e-mails, many are trying to provide a far more multimedia experience, instead of just sending text down the line which takes up very little, many are sending image data, audio data, and video data so that can all cause slow downs.
There isn't just devices in your neighbourhood you have to think about, there is also devices in your own house as well. All these if they are connected to your router are all taking so much away from your internet speed. This time you are making your own contention level in your house.
E.g. in our house we get 2mbps from our line I have a smartphone, my laptop, my desktop, my PS3, my Xbox 360 so already that's 5 devices all sharing that 2mbps connection, then adding to that we also have an interactive TV box, Wii, VOIP phone, my sisters laptop, my mums laptop so yet another 5 devices, so if all 10 devices were to use the internet equally flat out we'd each only get 0.2mbps.
As well as more people connecting to the exchange in your area slowing your internet down you can also face the same problem with over usage of the radio spectrum in the area. Where we are we're lucky as we have 5 houses in our area, and out of the 5 houses only 3 have wi-fi. But in some really built up areas you may find you have 20 houses and when you do a scan of wi-fi in your area you may find you have 20 routers as well. Each one of those routers needs a specific frequency to operate on, whilst one router is sending data on that frequency another router has to wait it's turn to send data or else the two will conflict. Kind of like when you go into an area that has two radio stations on the same frequency, you can't hear either one because it's constantly switching between the two - in Manchester, UK - Key 103 and Kiss 102 are really good at doing that due to how close the two frequencies are. So if your using wi-fi you will find that the more routers in the area, again the more slower the internet will be. The best solution in this case would be to invest in an 802.11N router and 802.11N wi-fi cards for your devices, or look at sending the internet through the mains sockets instead of using wi-fi.
So as you can see there can be numerous reasons for a slow internet connection and pinpointing it can be a nightmare, one more final thing as well is that if you are on ADSL and not fibre optic broadband check your ADSL filters as sometimes they can go and they seriously affect speed when they go, you can normally tell if your ADSL filter has gone by picking up the phone that is connected to the filter (if there isn't one connect one to the phone socket and leave the router plugged into the other socket), then call someone, if you can hear a distinct high pitched tone over the top of the call then your filter isn't doing it's job). Also if your on ADSL ensure you plug your router into the first phone line in the house, do not use any extensions as this can cause problems with the internet as well.