Your next step should be to check for spyware, at least to eliminate the possibility.
Download HijackThis from http://209.133.47.200/~merijn/files/HijackThis.exe & unzip it into it's own, permanent folder, not a temporary one. Start HJT & press the scan button. When the scan is finished the scan button will change to save. Save the log to a text file & paste it into the body of your post. DO NOT FIX ANYTHING YET.