first thing is delete all your temporary internet files.
(on XP - open internet, goto Tools, Internet Options, Delete files).
Then ensure it's not just slow because it's trying to download Windows updates - they can really slow things down, depending on your system. If they are, let them finish.
Try spybot or similar to check for malware - can't be too careful (free download)
Then try defragging your system: Start -> All Programs -> Accessories -> System Tools -> Disk Defragmenter.