Scan your system for Spy-Ware, these small bits of software are sometimes missed by the antivirus, and there is a high chance of spyware being the cause of your password being stolen. If you have any other privacy software RUN that too, check if you have a fire wall and that its set up properly, always scan files before you run them. Using browsers with built-in firewalls, a router's firewall, and the regular firewall on a computer helps you to prevent many problems.