First off, I totally agree about the Mac suggestions - I do think that the Mac OS tends to be more stable than Windows (even XP). Having said this, I just took my MacBook to our local Apple Store to swap out a defective battery (wasn't being recognized by the computer) so Macs DO have problems on occasion as well.

But getting back to the matter at hand ... I agree that it could be a virus/spyware issue but my personal bet is that your Windows installation has become unstable from conflicting startup items - including items that you can't deactivate on startup. Sometimes these conditions can be fixed by software tools like the one that Karen mentioned above (hey, it's free, it's worth a try) or something like Norton WinDoctor (part of Norton Utilities). If all else fails, I'd backup your documents to an external hard drive and reinstall Windows from scratch with the disks that came with your computer. You may notice that things "zip" a lot more than they used to, not just in startup but in general usage as well.

Sorry - it's usually a painful process to fix these problems. :(

Masaki