Had the same problem. Contacted Steam support. This is what they told me and it worked. -If you are running in Windows Vista or Windows 7, please completely exit Steam, and right-click on the shortcut which you use to launch Steam and select "Run as Administrator." -If you are running in Windows XP, take the following steps (ignore these if you are using Windows 7 or Vista): 1) Exit Steam. 2) Go to the "Start" menu, select "Run", type "regedit" and click "OK". 3) Navigate to the following registry key: HKEY_CURRENT_USER\SOFTWARE\VALVE 4) Right click and select "Permissions" 5) Check the "Allow" boxes for "Admin" and "System" for both "Full control" and "Read". Then verify the GCF for this game: Title: Verifying Game Cache Files (GCF) Link: http://support.steampowered.com/kb_article.php?ref=2037-QEUH-3335 Hope this helps.