system
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If the usb got infected, it could be scrubbed also. You would just want to be careful not to use it in the interim.
Anyway, this is the resident malware remover’s guide>>http://forum.avast.com/index.php?topic=53253.msg451454#msg451454
Once you have read it over and run the tools requested there, post the resulting logs back in this thread as attachments and wait for him to read them over and request what to do next.
also, your email is showing public on the forum.