|Had the same problem - vista was infected with a virus - Vista security ...something.. that didn't allow me to go anywhere after login, pretending to be an antivirus scanning HD.|
There was an advice to restart PC in a safe mode and run Malwarebytes, but the same virus was active even in a safe mode and didn't allow me to do anyting.
So extracted HD, connected it to another PC using SATA/USB adapter. Sanned it with Malwarebytes, then installed it back. This time I could go anywhere on the laptop, but couldn't run any exe files. I couldn't open cmd or regedit.
So what I did, I open explorer using "run as administrator" (go to Start/Programs/explorer, right-click on a explorer and choose "run as a dminstrator". Then I downloaded CCleaner and Glary Utilities. Both are free SW. Installed and ran both of them (again, run as adminstrator). This corrected registry problems and everything started working again. I don't know which utility helped.