Infected with the Firefox mysearchcorp redirector?
Try this in Firefox... Tools -> Options -> General -> Manage Add Ons -> Extensions -> Firefox Security -> Disable
I am no expert but I am stubborn. I spent all day with no luck, checking the registry, running HiJackThis and telling it to fix suspicious settings, and even running full scans in Trend AV, PCTools, and StopZilla. Finally I resorted to trial-and-error, renaming folders and executables, stopping services, disabling plugins and extensions, and lo-and-behold, this fixed it. When I enabled it, the problem returned. So then I uninstalled the Firefox Security extension. Now everything is fine.
Please let me know if this works for you too.