Hello sander00171,

Sorry to hear about your trouble.

Please try out the following steps:

1. Start Steam and right click on DayZ in your game library (make sure that the game is not running).
2. Select the tab labeled "Local Files".
3. Click on the button labeled "Verify integrity of game cache...".
4. Restart your pc once the process is complete.

After this, if the problem is not solved, do the following.

1. Download and install Microsoft Visual C++ 2010 Redistributable Package: http://www.microsoft.com/en-us/search/result.aspx?q=vcredist%202010&form=DLC
2. Restart your pc.
3. Download and install Visual C++ Redistributable Packages for Visual Studio 2013: http://www.microsoft.com/en-us/search/result.aspx?q=vcredist%202013&form=DLC
4. Restart your pc.
5. Launch DayZ (run both Steam and DayZ as administrator if needed).

If this does not solve the problem, here's a suggestion that is a bit more drastic. Please be aware though; you might have to reinstall your other Steam games afterwards:

1. Move the entire "C:\Program Files (x86)\Steam\SteamApps" folder to your desktop. Your "SteamApps" folder should now be at the following location: "C:\Users\<username>\Desktop\SteamApps".
2. Move the "DayZ" folder out of the "C:\Users\<username>\Desktop\SteamApps\common" folder and subsequently delete the "DayZ" folder.
3. Uninstall Steam while making sure the "SteamApps" folder is still on your desktop.
4. Restart your pc.
5. Install a new version of Steam.
6. Go to the new "C:\Program Files (x86)\Steam\SteamApps" folder.
7. Move everything inside the old "SteamApps" folder on your desktop to the new "C:\Program Files (x86)\Steam\SteamApps" folder.
8. Install DayZ again.

Regards,
JStewart