Serious Sam HD: Second Encounter (HELP!)

So, the problem is this, that once I installed SS HD: Second Encounter Demo and played it for a while (really, out of boredom). But now (well, actually it's been a while, but now I just got myself to write this) SG shows that this game is in my inventory, even though it isn't - it was a demo! So, I don't know how to fix this, because I can't join any SS HD:SE giveaway, and I know that there are some. Please help! ;_;