My computer was host to a multitude of virus's which left my computer barely functioning.  Eventually, the start menu was shot and the desktop would freeze up if you tried using it, and my computer would go into a blue screen and state that a fatal error had occured.  Seeing as how I do not have a restore cd, I am wondering how I would erase my harddrive and start from new.  I have already tried reinstalling Windows XP and it TOLD me that the version on the cd was earlier than the version on my computer, and I haven't been able to find a later version of the cd.   Eventually, my computer started to stop functioning at sometimes, stopping at the BIOS screen, so is there a way to run from the CD directly from the startup?  Is there a way I can ENTER dos mode or the command prompt from the startup of the computer?  Please be detailed in your explainations from start to finish.  Any help would be GREATLY appreciated.You need to get into your bios, usually by tapping the Delete key at bootup (your key may be different), and set your cdrom as the FIRST boot device.
Then boot with the xp disk in the cdrom.