If your PC did not come with a complete Vista or Win7 installation CD, you can download a Recovery Disc at the following links:
Windows Vista/7 Recovery Disc - for repairs only
http://neosmart.net/blog/2008/window...disc-download/
http://neosmart.net/blog/2009/window...-repair-discs/
Vista will not repair XP(they create the boot sectors differently) but can run check disk.
http://support.microsoft.com/kb/927392