How do I delete a virus that wont let my anti-virus program start?

The other day I downloaded an .exe file, after launching it, it disappeared in front of my eyes and a fake looking spyware protection program opens up, makes this horrifying beep noise and says its scanning for viruses, and say's its finds 6 every second, and when I go the the task manager it shuts it off so i cant stop it but sometimes i can manage to shut it of from the task manager before the task manager shuts off, And if I try to launch my anti-virus(AVG) it wont let me and a little balloon from the start bar with a little logo of a shield pops up and it says something like (your anti-virus is restricted to open due to a probable virus on the computer (and it has the same logo as the .exe did)also it wont let me run any .exe files like games ect, including system restore:( What do I do?