When you say its not your firewall, which one do you mean?

On the PC? Your primary site or home router firewall?

Can other PC's log into secure sites on the same network?

Tried it with your antivirus disabled?

Have you run antivirus scans lately?

Run Spybot Search and Destroy

Run a free online virus scan from http://www.bitdefender.com/scanner/online/free.html