You've infected yourself with some very nasty malware...
There's only a couple of options for you at this point and you should follow Keith's advice on the Repair installation as the other option is to format. Once you have the machine running...
Please go to the Malware Forum
and follow the instructions at the top....Especially the Start Here
That will give you several steps that will help you clean up 70 percent of all problems by yourself...then post a hijackthis log in THAT
forum. Be patient, the Malware Forum is a very busy place and a two or three day wait is not unusual. DO NOT REPLY TO OR BUMP YOUR OWN LOG. If it shows a reply it may be overlooked as one that is being worked on.
If you are still having problems after getting a clean bill of health from the malware expert, please return to this thread.