Do you want to try and clean the machine or format and reinstall windows?
If you want to clean it this post belongs in
Computer Viruses and SpywareOver there we have posts on what steps you should take to help our malware experts help you.
If you want to format and start over please provide us with a little more information. Like the system specifications or brand name and model of the computer. We will also need to know what version of windows you're trying to reinstall.
I can tell you though that a virus can't prevent you from booting an Windows install disc. Something else is causing that problem.