It was working perfectly fine this morning before I put new motherboard etc, in.
Sure you can reinstall Windows, and it might be the simplest fix.
However, if it was running fine then boot off the Windows DVD, press F8 until the 'Repair' screen appears and run System Restore, failure you have choices: Startup repair and lastly Reinstall Windows. If you reinstall Windows then run Windows update -- you might need to reinstall Apps.
What changes did you make to Windows i.e. installation, updates, drivers --- maybe a virus?