First, turn off all of XP's unnecessary services. There are numerous guides online for which services to turn off, which to set to manual and which to leave on automatic but I've found
Black Viper's site the easiest to read...and the links to explanations of each service are handy. It takes a little time but it's well worth it in terms of better system performance and online security.
Next, turn off all the programs loading on startup that you don't need to load when Windows starts...usually indicated by the presence of their icons in the System Tray. Disabled their loading on startup within each program's Preferences/Options/Whatever they call it if you can (reason: more and more software is being written to override msconfig settings as more and more users learn about msconfig).
For those that don't offer that option, go to Start>Run, type in "msconfig" (without the quotes), click OK. Go to the Startup tab and uncheck anything you
are absolutely certain you don't need loading on startup. Click OK. You'll be asked if you want to reboot. That's your choice but when you do, when Windows starts up again, you'll get a Dire Warning about starting in Selective Mode. Yes, you selected things not to load on startup so, yes, you want it to start in Selective Mode.
Then, if you haven't already, get rid of any spyware you may have picked up. Download, install,
immediately update and then run
Spybot and/or
AdAware. Read the help files to familiarize yourself with how they work. I recommend you use them both since each tends to find things the other misses. Let them clean up any spyware found.
Then, to be doubly sure, download and run
CWShredder (link to the download is near the bottom of the page).
Then finish up by running Disk Cleanup (Start>All Programs>Accessories>System Tools>Disk Cleanup) and then run defrag (Start>All Programs>Accessories>System Tools>Disk Defragmenter)
All of the above with the exception of eliminating programs you don't need from loading on startup, should be done as part of regular computer maintainence. I run Spybot and AdAware weekly, disk cleanup whenever I think of it, defrag monthly.
Be aware that if you have Norton Anti-Virus and it's configured to run full-time in the background that it's a resource hog and can have a major impact on system performance.
If after doing all that there's no improvement, post back.