The easiest way I can see to fix this problem is to reinstall Windows. The Easiest way to do this is to use the recovery options provided by the computers manufacturer(ie. Disks that came with the computer when purchased). Oftentimes the laptops manufacturer also has alternative recovery options available. You can find options specific to your computer but googling it. Most manufacturers offer some sort of recovery partition on the Hard drive that can be entered by pressing a specific key upon start up of the computer. I personally have found Google very valuable in situations like this.
AS AN ABSOLUTE LAST RESORT FOLLOW THE INSTRUCTIONS BELOW AT YOUR OWN RISK.
Windows can be installed completely independent of the computers...