[MuGen]

Reboot with F8 in Safe Mode,

Start -> Run -> type in 'cmd' and a DOS prompt should come up

Type 'sfc /scannow' without the quotes. This utility checks Windows 32 system files and makes sure they are in tact and in their original state.

If any has changed insert your Windows OS CD that came with your computer to copy the original files back.

If the problem was solved, then it was Spyware/Adware...