Mafia 2 crashes after startup

Hi!
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 used to change this computer?" (excuse my bad translation from swedish to english).

Programname: Mafia II Application
Verified publisher: Valve Corporation
File 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.

I run on Windows 7 Home Premium 64 bit.

Hope to get some help here and I do apologize for my bad english!

EDIT: My Specs:
OS: Windows 7 Home Premium 64bit
CPU: Intel Core i5 CPU 650 @ 3.20GHz
Motherboard: Gigabyte P55-USB3
Memory: 4 GB
Graphic Card: ATI Radeon HD 5500 Series
Sound Card: Realtek High Definition Audio
Screen Resolution: 1680 x 1050 x 60 hertz

Glad you got it sorted out.
For future reference, that little trick will fix a large portion of issues with a Steam game failing to launch or update.