Yes, demigod was published by Stardock, so it should work without any problems. It's how I got my copy of demigod.

If someone is selling JUST a key (as in no box or disc), and it's not through impulsedriven.com, it's probably not legitimate.  If a retail store is selling a legit copy of Demigod, it will be in a box, still sealed.

Be aware of where you purchase your keys. You're more likely to find an illegitimate copy of a product if you purchase it from an obscure online retailer. As a suggestion to minimize frustration, it's best to buy from either Stardock directly, or from a major retailer (Best Buy, Walmart, etc). Also be sure what you're buying is not used (because it may be already registered to another individual) -- no retailer should not be selling used copies

Mike