Unfortunately, reinstalling Windows doesn't necessarily destroy virii.
Formatting your hard drive is what will get rid of them.

Formatting your drive can be done in a number of ways, and unfortunately, is a fairly non-trivial exercise.
My personal preference is to use a Live CD for a Linux distribution and run the program "cfdisk" from the command line, however, having said that, cfdisk is not the easiest program to use.

I believe you could use a graphical partitioning tool such as Partition Magic to do it as well, which is a lot easier.
Just be absolutely sure to back up anything you might need, because formatting will destroy the data on your drive, and to recover it you would need to pay a large sum of money to a computer forensics company.