Hello... Do a re-install of the originals CD's/DVD"s to your computer... but... you may or may not lose all your Personal information/DATA... depends on the OEM install Disks. It is possible to re-load windows only in some cases... over your system without lossing anything... But depends on your options with the OEM disks.
There is also a "restore option" in Windows, to an earlier date... results can vary depending on if restore points were made and saved.