Mafia 2 Crashes After Startup?

I've been playing Mafia 2 since it came out.I installed it (after buying a DVD copy) and linked it with my steam account.I played it and completed the game, but now after about, 1 month without playing the game, I try playing it again, but it wont start.I click on the game in the steam games library and it starts "booting up", and the "User Account Control" window pops up saying something like "Do you allow this program to be ud to change this computer?" (excuse my bad translation from swedish to english).Programname: Mafia II ApplicationVerified publisher: Valve CorporationFile source: Harddrive on this computer(The game itself is stored in the steam folder under "Program Files (x86)" )The options are Yes & No, however, if I press "Yes" the window just pops up again, and again and again and so forth.. However, sometimes it just gives in, but still nothing happens...As I said before, it worked properly before but now it wont even startup, not even the "startup" screen and crash.. NOTHING..After going through different threads on the internet, both on Steam forums and 2K forums after a solution I find nothing but people having the same or similar problems, but no solutions