Hello guys, I would just like you all to know that everyone having this problem should submit a ticket to steam, THQ And the humble bundle support if that's where you bought it from... I also highly recommend looking through the titan quest forums.http://www.titanquest.net/tq-forum/threads/45443-Humble-bundle-CD-KEY-NO-VALID
^ I have made a post and for people who would like to sign up on the forum and bump it up so it gets some more attention that would be great.. I am personally working very hard on trying to fix this problem and will hopefully come out with a fix for everyone with this problem.
FIX FOR SINGLEPLAYER : Go into reg edit and press ctrl+f And type in IRON LORE, after that in the navigation panel go into the Titan quest folder and in there you should see a registry called Cd key or something along the line of that, Delete it and then verify your game files and start the game, This should fix it (This has been tested on windows 7 64 bit and windows 8 64 bit, How ever this hasn't worked for everyone)