Yeah for Wiundows 7 you don't need original install discs. The licence is the sticker on the machine. Just download ISO and create a DVD or USB to reinstall. You'll need to get the device drivers separately from Dell though.

Direct download of ISOs here:
http://www.howtogeek.com/186775/how-to-download-windows-7-8-and-8.1-installation-media-legally/

An install "over the top" or repair might fix it. Image it before so you can roll back if necessary.
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Christian
#40104192 120Gb (no longer in my E36 M3, won't fit the E46 M3)