The first thing to do, is try to remove all infections. Boot into safe mode with networking (F8 on startup). Download, install, and update malwarebytes. Then do a full system scan.
As tominfl has experienced, sometimes the damage has been done to system files. In a case like that, the easiest fix, is to do a windows repair. This process is similar to a windows install, but it simply replaces system files. It won't effect user data at all. Here's a guide on how to use the startup repair.http://www.bleepingcomputer.com/tutorials/tutorial148.html