I had a very similar issue recently. It wasn't a virus, but some badly misbehaving software. I would suggest you run msconfig and turn off everything at startup that you're not sure you 100% need immediately.
If this cures the issue you can then re-enable stuff gradually to diagnose exactly what is giving you the issue.
If you're really convinced it's a virus then maybe MS Onecare's online scanner could help. http://onecare.live.com/site/en-us/default.htm
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