To eliminate any chance it is being caused by malware, please go to the Malware section
of this forum and follow the instructions at the top. Especially the Click Here
link. That will give you several steps that will help you clean up most of all problems by yourself. If at the end of the cleanup process you are still having difficulty, and you may not be, then post a hijackthis log in THAT forum.
Please be patient as the "Malware Forum" is a very busy place and a two or three day wait is not unusual.
If you are still having problems after getting a clean bill of health from the Malware Section, please return to this thread. Please do not post a HiJackThis log in this thread unless asked to do so and then only as an attachment.
To attach log, when requested, either change the .log extension to .txt, or put the log into a compressed (zipped) folder.
Please post back if your problem has been solved.