to check for nasties:
try installing this: http://www.malwarebytes.org/mbam.php
once installed, let it update then run it
Can a virus infect one account while ignoring another. Nothing has been downloaded, and a system restore to two days previous has no effect.
maybe you got a mailer doing mass spams from your side?
anything on your side not running on the other?
do you use anything like Process Explorer? You'll see more stuff running in it than in task manager.