Reinstalling Windows does not require that you reformat the HD, unless you're having a FAT32/NTFS issue. The reinstall should place the old windows in a folder called, logically, Windows Old (or maybe it's Old Windows, can't remember). So if the internal HD is okay, and if you've got a few gigs of space on it, you should be able to get it up and running with no problem. Just don't format it.