I won't even go into my normal rant on what a piece o crap vista was. If you you are still running vista you need to upgrade. In the meantime Microsoft has a pretty good link on this issue.
http://windows.microsoft.com/en-us/windows/startup-repair-faq#1TC=windows-vista
http://windows.microsoft.com/en-us/windows/startup-repair-faq#1TC=windows-vista