I had the same problem and fixed it!
I also run WinXP on a Pen III 800Mhz, 133Hhz bus and I have just over 600MB SDRAM. So, how did I fix the problem?
Background. The more programs you install, the more they tend to RUN quietly underneath. This eats at RAM and Processor. You image a crowd of 1, 000 people after a football game rushing to get through a narrow set of double-doors designed for two at a time, you get a bottle-neck, that's the way your pc is at Start Up when its not got the power. Imagine now 50 people instead in an orderly line walking through the doors, a lot quicker and easier.
Here is an example of programs that RUN at Start Up, say after a month of downloading alot of free & payed stuff:
QuickTime, iTunes, Real Player, LimeWire, eDonkey, Internet TV, MS Messenger.exe, Yahoo! Messenger, Yahoo! Toolbar, Skype, eBayToolbar (Internet Monitoring not browser toolbar), Nero Burning Rom monitoring tool, Windows Media Player, System Mechanic 5/6 monitoring tool, Adobe update, Bluetooth Device, Nokia PC Suite, GoogleToolbar, GoogleAccelerator, C-Media Audio, your Modem or IP stuff, AV and all the other freeware and payed stuff and so on.
Some may have adware (or spyware if its not from a trusted site or company) that came with it and each, lets say, asks Windows to RUN at Start Up. The more that ask, the more clogged the system gets the more your PC slows down because its not powerful enough to run all these at once and do every day tasks. However, you now can have the option of not having most of them starting at Start Up. You have to tweak the registry in order to do this unless those programs give you the option of not starting 'when Windows Starts' - but still tweak just in case.
Lets start at the beginning where everything boots up, that is the Registry. On your desktop taskbar click 'Start' and 'Run' then type in 'Regedit' and the registry opens up. Next go to HKEY_CURRENT_USER/ControlPanel/Desktop and reduce the values in MenuShowDelay & HungAppTimeout and the other WaitToKillAppTimeout to the lowest like 0-500 by right clicking on the icon then 'Modify'. Do keep up! Then once that is done you need to go to HKEY_CURRENT_USER/Software/Microsoft/Windows/CurrentUser/Run now this is where 1 or a few programs are registered to run at Start Up. Do you need them to run as soon as your pc starts? That's what you have to ask yourself.
Next its HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentUser/Run and check the programs that are due to run. There you will see things like Symantec or another AV, leave these. But, you may see QuickTime, iTunes, Real Player and other strings wanting to monitor constantly. Do you really need them running 24/7? If not, delete the string value (the whole file) and so on until you have running only the most important programs you need to start running your pc. Then when you want to run the programs you can only when YOU want to.
Again, automatic updates, I know this will get some people's back up but if all the auto-updates are running they will all want to update as soon as your PC is running or as soon as you logon to the Internet. If you must have auto-update then just your AV and set Windows Update to manual only to ask YOU when Windows should update and if its convenient to install.
So, instead of keep running Task Manager everytime, now you can go to the source and stop most of these programs from quietly running underneath at the root or in this case the Registry. Then, when you want to use and update them you have the control.
If you run SpyBot S&D and delete adware called 'doubleclick' then Firefox will not run properly as its part of Firefox default. Seeing that it is 'freeware' and isn't really free because the price is this adware, then by deleting it you may be breaking the End Users Licence Agreement. When I ran SpyBot S&D I kept on deleting 'doubleclick' and wondered why Firefox was running slow or not at all, until I realised it is part of why Firefox is free.
Remember to delete 'cookies' (as some of these can be adware/malware/spyware which can cause system slowdown) as soon as you have finished surfing the Internet. Tools/Options.../Privacy/Cookies and delete them. You may wish to spend time adjusting the settings to the way you want Firefox to run or any other browser. You may like to clear Private Data (online form filling), cookies and Temp Internet Files cache as soon as you close the browser.
I hope this helps you, you can also download from this site FreeRAM XP pro that will regularly free up RAM to help your PC to keep running smoothly. Next is regular defragment of the Hard Drive. Which is basic to all PC owners. Sart/Control Panel/Performance and Maintainance/Rearrange items on your hard disk ... while you are there click 'System' and see how much RAM and what CPU you have and adjust Windows Automatic Updates.
You now have control back of your PC. Also, another good program is Ad-AwareSE which you can download from this site.
Another thing, is that programs get upgraded through updates and use more RAM & CPU. New PC's can handle this with larger RAM and greater processing power. So, beware when going for the latest program, make sure you read all the specs and find out how much it will use when running on your machine. Yet, KasperskyAV uses less CPU & RAM and works quicker than some popular bulky AV programs.
Make sure you have just and only what you need and what you want when you boot up, as you don't have the capacity or power to have them all running at once. Some programs can clash at Start Up.
Don't lose heart at the Spock tech bits though, learn all you can and enjoy your PC & Internet surfing but be wise, use McAfee SiteAdvisor with your browser as well from this site.
When a pc slows up it can be a number of problems and so you have to go through different diagnostic steps to see what it is. Don't get frustrated at your AV when you need to defrag or hit your mouse on the head when you need to hit 'delete cookies'.
The best anti-virus tool is between your ears!!! Be wise, be safe, have fun!