There are a number of ways to increase startup speed on desktops and laptops.
For a bit of a speed boost, change the way Windows shuts down. You may find that your newest computer that takes a full 2 minutes to boot up may be set to "shut down" when either the lid is shut or power button pressed, where as the laptop that takes 45-50 secs maybe set to "hibernate" or even "sleep".
If you set the computer to sleep when it's turned off this can dramatically reduce start-up times because the computer doesn't actually turn off, it just reduces power to non-essential devices, such as turning the monitor and hard drive off, slowing the CPU down and just keeping it ticking over, slowing down or stopping the internal cooling processes - with the CPU running slower it doesn't get as hot so doesn't need as much cooling, etc.
If the computer is in sleep mode it can start up in a matter of seconds as all it needs to do is just switch the power back onto everything and get everything up and running again really fast. All the previous data is all still stored in the RAM and so it starts up very quickly.
The only problem with sleep mode is that if anyone catches the mouse or keyboard or sometimes internet messaging can bring the computer to life. This can be quite a nuisance if you put it in sleep mode and someone sends you a nudge through MSN messenger at 3am straight through your sound system - grrr...
The next stage is hibernate mode. Hibernate mode is a little slower at starting up than sleep, this is because Windows does actually turn everything off. But before it does this it takes a snapshot of the RAM and copies it to the hard drive - if you've ever played emulator games and you get like ZX Spectrum SNA or Z80 files, these work in a similar way - and with the SNA files you skip the entire tape loading sequence to load a game, the same way Hibernate speeds start up times.
In Hibernate mode the PC fully switches off, but when you need to turn it back on Windows just copies all the data that was in the RAM back into the memory (as well as a few other things, so it's not quite as fast as a ZX Spectrum snapshot). But by doing this it isn't having to go through the start-up processes of all your software - such as your anti-virus software has an entire startup process, Office has a startup process, Adobe has a start-up process, and so on - all this is skipped so it makes the boot a lot faster. Plus in hibernate mode you will find that whatever you had open when you turned your computer off is still there, ready and waiting for you, so if you left a document open you can just press the power button and the document is there ready and waiting for you to edit again.
Sometimes with hibernate mode you may find that it can take longer than re-booting your computer to startup, this is due to leaving a problematic program open. Some programs do not work very well with hibernate mode and should be shut down before entering hibernate mode. The majority of programs are very well behaved and don't mind being having their entire data copied to the hard drive and then brought back, but some, especially those wrote before the days of hibernate can have some issues.
Of course it's not just startup modes that affect the speed, the hard drive, memory and programs can also affect speed.
If you really want a fast start up time replace the hard drive with a SSD drive, SSD drives are far more faster than standard drives because they have no moving parts, no disc, no magnetic head, no spin-up time. It's like if you look at the speed that something like a Sega Megadrive could load games from cartridge in and comparing it to the speed it took for the MegaCD to load the same game (I know the megadrive is very old, but it was the only console I could think of that had both cartridge based software as well as disc based software without going as far back as the C64GS)
Also with SSD drives because they have no moving parts they are a lot less prone to failure, although that doesn't mean they are totally infallible, the only major issue with them is that if a drive fails with moving parts you can quite often recover some data, usually though when an SSD drive goes, it goes and takes everything with it, so make sure you keep regular backups.
Then you also have RAM to consider. The more RAM the faster your machine will load, with one exception - if you are using a 32-bit version of Windows do not put anymore than 3.99Gb of RAM inside the machine (so RAM 4Gb and over use 64-bit Windows), I'd highly recommend using 64-bit Windows anyway regardless of whether you have that amount of RAM. I've only got 2Gb in mine and it runs 64-bit Windows Vista far better than I've seen similar computers attempt to run 32-bit Vista.
Also if you do switch your OS from 32-bit to 64-bit make sure the CPU does support 64-bit, as your computers are all only a year old they should really all be 64-bit CPU's.
Another thing to consider for a faster boot-up can be the GPU within the computer, if it's a laptop you will have little or no choice about this. My laptop has an AMD Mobile Radeon within it, with dedicated memory, but most laptops just have Intel graphics chips, with shared memory. If it's a desktop PC and it's using onboard graphics with shared memory, try adding a separate GPU card with it's own memory. This will reduce the burden that the machine has with the memory, plus Windows will be able to pass some of the tasks of starting up the Window 3D Aero interface onto the dedicated GPU rather than hogging the CPU with it. Years ago the Amiga's were really fast for numerous things and this was because they had the main Motorola 680x0 CPU at their heart (which had hardware multitasking), and it had dedicated chipsets around that taking the load off the CPU - such as Agnus (memory CPU), Denise (GPU), and Paula (audio CPU).
Finally the other thing to look at is your start-up programs. Run MSCONFIG and look through exactly what is starting when the computer first loads.
Each of these programs all slow the computer down when loading and can lead to longer boot times. Remove any programs that aren't necessary, such as "Quickloaders" that are supposed to pre-load parts of programs such as Office and Acrobat into memory, ready for use when you need them. All these take up processing time on start-up and can slow boot speed down. You also have "autoupdaters" these load on startup and check that you have got the latest program there and then and if not go and download the new program, again at times you may be better just disabling these and manually updating the program once a week, of course some of them are important for security such as anti-virus updates and java updates should not be disabled. Also when looking through the loading section keep an eye out for any strange programs especially ones that are just made up of random hexadecimal numbers and are in strange places such as A4433BG4G999.EXE these can quite often indicate some malware or virus that your antivirus has failed to pick up on.
I also notice that you mention that you use Norton for your anti-virus. Norton isn't too bad as an anti-virus, it's certainly a lot better than others, however Norton is a bit of a "bloatware" piece of software, in the fact that it does use an awful amount of system resources for the job it actually does. This could be one of the things that is slowing the PC down on boot up, and there are other virus killers out there that you may want to try, like ESET Smart Security or Kaspersky Internet Security and see how they fair speed wise in comparison to Norton, you may be very surprised.