I had this same problem. I did this:- Open the registry editor (Start, Run, regedit)- Go to "HKEY_CURRENT_USER\Software\Valve". Delete the folder "Half-Life".- Now go to "...\Steam\steamapps" and delete "half-life.gcf" and "half-life engine.gcf"- Go to "...\Steam\steamapps\your account\half-life" and delete everything but the folder "valve\SAVE" and files you might be interesteed (such as screenshots and configs).- Delete everything in Steam's folder except "steam.exe" and the folder "steamapps".- Restart your PC and reopen Steam.- Download HL again and play.