IF you can access the Internet without major problems, I would advise going to McAfee's or Symantec's websites and running their online scanners. If they don't find anything, then I'd say (without having more info) that you need more RAM or your PC needs to load less items at start-up.
If the scanners find something (trojan, virus, etc), go to downloads.com and get Spybot Search & Destroy v1.4, Ad-aware SE Personal, Spyware Blaster v3.5.1 and AVG Free v7.5. Install ALL and update them.
Next, DISCONNECT from the Internet and run a scan with AVG. After that, run a scan with Spybot. Last, run a scan with Ad-aware. Delete EVERYTHING they find using the respective program's delete/fix function.
Lastly, reboot and see how your PC "acts". If it's still slow, come back and let us know. I have other options that can help you at that point.
CharlieJ
April 2007