I got curious about the mp only key, and looked it up. Apparently they can be received for free through a polish website.
You can read up on it if you want, found a few forum threads with information.
http://thebotnet.com/freebies/36134-fre ... might-and/
http://www.facepunch.com/threads/1018615
Apparently it was promotion in Portugal, seems odd since all other sources say it's a polish website. Maybe he meant Poland?
Steam forums: "wow the free dark messiah promotion is still running? that was years ago. should be technically legal, it was originally coming from a game magazine in portugal, nothing stolen or anything"
Also Steamgifts.com lists it as a exploited key:
Keys Considered Exploited:
Serious Sam: Double D
Dead Island
Bulletstorm
Homefront Shotgun DLC
AI War
Portal 2
DiRT 3
Dark Messiah of Might & Magic Multiplayer
Eets (in certain amounts)
Don't really know what to make of this, but I hope this will help if someone else gets the same problem. Also, I backed up my saves and I had no problems, so I didn't suffer by the reinstall. Yet again, Thanks for what you have done for the community