Steam won't install game

[gabecube]

I bought King's Bounty on Steam, but when I run it, it always gives me the "First Time Install" message, "Step 1 of 1", closes the window and nothing happens.

When I try to run it again, I always get the same result.

I am running the Windows 7 RC1 (build 7100), though I am an administrator and I disabled UAC completely.