When you say you "reinstalled" windows, was it a Repair Install or a Fresh Copy? Prior to this problem coming about, did you install any new hardware? Did you have up-to-date antivirus software installed and running?
My initial inclination is to try and rule out newly installed hardware conflicts/issues and a boot-sector virus.
My initial inclination is to try and rule out newly installed hardware conflicts/issues and a boot-sector virus.