format c: will remove everything from that drive...though you can't run that command from within windows...
if you format the drive in any way..with fdisk from a boot disk...with an os cd..with dban...anything....you will lose everything on that drive...if your friend doesn't have all of his program cds...it's in your best interest to try to clean the computer instead of formatting it...otherwise he/she will have to buy the software again to install it
Please go to the malware
forum in my signature and follow the instructions at the top....Especially the CLICK HERE
That will give you several steps that will help you clean up 70 percent of all problems by yourself. If at the end of the process you are still having difficulty--and you may not be-- then post a hijackthis log in THAT
If you are still having problems after getting a clean bill of health from the malware expert, please return to this