How do I uninstall Vista?

My dad’s laptop has a virus called “Vista”. It looks just like an OS, something like XP, except that it’s crap. So I want to uninstall it and replace it with a real operating system instead (DOS, Linux, Windows 95… anything but this crap).

The problem is that the computer sees the installation CD as a potential threat and won’t let me either downgrade (via Vista) or install (in the boot-up sequence) or in any way touch the hard drive. I cannot even reformat or partition it, as would normally happen with a WinXP installation disk, because the system freezes when it loads the drivers. I think it’s doing that on purpose, there’s nothing wrong with the WinXP CD (it’s an original copy).

The best advice I could find so far is this http://www.tech-faq.com/how-to-uninstall-vista.shtml
which is exactly what I’ve been doing all along anyway. It doesn’t say how else you can get rid of this “vista” thing if it won’t go away. All I need is some way to at least go into DOS and format C:, but even when I found a command prompt (using a Win98 CD) it could not find a “C:” drive, or any other letter for that matter (except the virtual drive A: and the floppy B:).