When reinstalling Windows XP to replace an installation with a corrupted registry, I have two options.
- Reformat the disk and start with a blank disk.
- Leave the original filesystem intact, so I can recover the files.
If you're talking about re-installing Windows (rather than just fixing/correcting the registry), I'd suggest backing up any files you will want to use again to some external storage (external HD, DVD, online backup, etc...) and then do a clean format and re-install. If you can't even get the current installation to boot, then either mount it as an external drive for a different OS, or use a Windows recovery boot disc (it's been a while since I've done this but there should be many such things out there).