Have you tried installing, the Window CD(DVD), that came with your Dell.
Mines red (the CD(DVD) cover), so I believe yours might be.
It's should say something like this on it.
Operating System
Already Installed On Your Computer
Also to reinstall it, put the CD(DVD) in your computer when it's loading up, then look for a message on the top right saying
Boot somthing(can't really remember)
Hope this helps