I doubt very much that installing Firefox would cause a problem like that (assuming you downloaded Firefox from an official Mozilla download mirror site), but I would be curious what caused the Firefox installation procedure to crash initially. If you have internet access with another PC my advice is to go to a Windows forum (like this one - http://lounge.windowssecrets.com/index.php ) and ask for help with trying to get Vista to boot before doing anything as drastic as re-installing Vista. I have never used Vista myself, but older versions of Windows have a Safe Mode that launches a bare minimum of services, and there's also a ScanDisk routine that can be launched.