One thing you can try is McAfee's online
FreeScan , which scans your computer remotely with a clean copy of VirusScan. In the case that you've got something on your HDD that is blocking your installed copy of McAfee, the web scanner will be unaffected.
More likely, though, is the possibility that whatever is going on is from spyware/adware rather than a virus. VirusScan can pick up some spyware programs, but that's not really its strong suit.
For that, you need a good anti-spyware program. Preferably more than one. My suggestions are:
AdAware (that's a direct link to the download site for the free edition. If you have trouble with it, go
here then look in the bottom right corner of the page for the link to download a free copy.)
SpyBot SpywareBlaster Download all three. Run scans with AdAware and SpyBot. Don't forget to update before scanning. Each is capable of detecting some things the other will miss. SpyBot works by searching for a specific list of known spyware and adware programs. AdAware works by checking the actual files on your computer and seeing if anything looks "wrong."
Hopefully, one or the other should pick up what's going wrong and be able to fix it. After that, enable SpyBot's resident "Tea Timer," which is the equivalent of VirusScan's "On Access Scanner." It stays active in the background, monitoring potentially dangerous changes and stopping spyware before it's installed.
Spyware Blaster passively blocks spyware from being installed. It doesn't even have to be actively running to work! Between SpyBot resident and Spyware blaster, you should be fairly well-protected from any new invasions.
(The makers of Spyware Blaster also suggest -- and I agree -- using
FireFox as your default browser, rather than IE. It has some really handy security features built in, and inherently lacks some of the larger security holes. For example, McAfee's FreeScan, which I mentioned above, is able to work through your browser to scan every file on your computer, looking for potential viruses. Now imagine what someone could do if they could abuse that power. There are some security measures in place in IE to keep that from happening -- and Spyware Blaster adds more -- but FireFox wouldn't let it happen in the first place. It means you have to use IE to do certain things like use McAfee's site or Windows Update, but it also means you're a lot safer when you're just generally browsing.)
If FreeScan, SpyBot, and AdAware don't come up with anything to solve your problem, though... I'm not sure what else to tell you. It may well just be a matter of Windows itself becoming unstable, and the only solution for that is to reinstall it from scratch.
So... Good luck, and I hope something here helps.
Paul