I think you need to do a start-up repair , From a disc , this tutorial from bleeping computers explains how
You may also not have a Full install disk , so this will do .
Nb. you do not need to use a torrent downloader for this disc just
double click on the down loaded link and select the browser of choice .
Some people have reported problems when they have burned this disk with Nero , I have had no problems with Vista's burner or Cdburnerxp ,
just slow the burn speed down a bit ,
If start-up repair does not do it for you try system restore .
A repair from a disc will work when sometimes it does not from a HDD .