I had a similar problem and it turned out to be a trojan horse virus. In the end I had to reformat my hard drive but before you do that there is plenty of other options like trying a good anti-virus package (f-secure) they do a free trial http://esd.element5.com/demoreg.html...4&languageid=1
repairing windows and I bet there is someone on this forum that could suggest something better than formatting your hard drive. I hadn't come across Tech Forums then but I wish I had, I lost everything. I know you reformatted a few weeks ago but have you got a virus checker/firewall running all the time?