Here's my usual take on fixing those pesky performance problems:
(Now don't tell me I'm being simple-minded cos you've done these things already, someone else might find it useful)...

1.  Antivirus (update and scan)
2.  Firewall (Gotta have one)
3.  http://security.kolla.de - Download spybot search and destroy, scan, immunize, fix all problems.
4.  http://www.lavasoft.com - Download AdAware SE Personal - scan, immunize, fix all problems.
5.  http://www.majorgeeks.com/download3155.html  - Download, install, and work through it 
     This one isn't for the faint of heart, but if 3 and 4 above don't get the junk out, 5 will.
6.  Dump your cache
7.  Purge your temp directory (c:\windows\temp), just kill everything over a day old.
8.  Defragment hard drive

If all this doesn't work, there are other things, but hopefully something like that will get you going.