If you have a lot of JUNK on your system and it all starts up at boot that is one thing alone that can make a high proformance system crawl. Check the startup list in msconfig. If you don't use it at boot or it's nolonger in use, uncheck the box. leave your antivirus and firewall checked. I find it's good to have them come on so I don't forget when jumping on line.
Next a handy dandy little tool is crap cleaner. A lot of here and at my place use this program to get rid of well crap. It works fine and is harmless to your system. www.ccleaner.com
Also a little used tool that comes built in to xp is the disk cleanup. Right click on the drive icon and select properties. Right below the pie is the button. I would advise putting a check in all of the boxes and running it.
Defrag. Boy some folks just putter along thinking everything is cool and then one day the system is slower than my granny. If your system has to look all over the drive to find bits and bytes of a file then brother the time it takes it to do that will make your scream. OK so go back to where you found the cleanup button and look at the top of the window. Click tools and select defrag. Now even if xp tells you you don't to defrag do it any way. It's lying to ya.
So to close, run your spy/ad/malware tolls in safe mode. Same with your antivirus software. AFTER you disable the restore points. Yeah and sorry to tell you when you do that it deletes them. But sometimes you have to do this if you know for sure you've got a bug. Don't forget to turn them back on after you're done.