It might just be a problem of basic maintenance. Here's a checklist:
-- Delete all temp files (c:\windows\temp\*.tmp, or on XP C:\Documents and Settings\username\Local Settings\Temp)
-- Delete temporary internet files (c:\windows\temporary internet files\*.*, or on XP C:\Documents and Settings\username\Local Settings\Temporary Internet Files)
-- If you use I.E., click on Tools, Internet Options, Delete Files, select "delete all off-line content", click OK
-- Click on Start, Programs, Accessories, Systems Tools, Disk Cleanup
-- Download
AdAware, check for updates, run it and remove whatever it finds
-- Periodically empty the browser cache and the java plug-in cache
-- Download
Diskeeper and defrag
-- Download, update and turn on
SpywareBlaster and
SpywareGuard (or your spyware removal tool of choice).
If that doesn't do it, then probably you'll have to start checking various components (besides what you've already checked).