Please be sure to do a registry backup before taking any of the following steps - gotta be safe!
Check your system tray for 'legitimate' apps that are sucking down your resources. CCleanercan really help keep an XP system clean by checking the registry, startup apps and program conflicts, etc (it will also give you a chance to backup any changes it's about to make).
I understand you're running AV and malware protection, but I recommend you run through this set of directions at MajorGeeks.com. Be sure to do the Basic Maintenance (defragging is vital and as they'll tell you, the included MS tool for defragging is far from the best out there) listed at the top before doing the rest of the guide and then be sure to follow the steps for Protecting yourself from Malware (this is near the very bottom) once you've completed the steps.
Even though you may have protection running, there may be nasties embedded in some things you DL and install that, since you're *installing* something, slip past the malware protection.
I can truly feel your pain. Until about a month ago, I was running a P4 2.0 GHz with 1 MB RAM. If you take all these steps and anything else suggested by others in this thread, consider wiping the HD and starting over. Not fun to think of doing but it will make a difference as you'll clear out a bunch of things that you really aren't using yet are dragging your system a bit.
