Not enough room? WTH?

I have a mac with BootCamp. I've been patiently waiting until Winter Break to fire up Civ V with all the new upgrades via Windows 7. I just fired up the PC side, and my Steam Library claims that Civ V is not installed. It's greyed out as if it has never been installed before. Which isn't true. I've played it a lot.

Then, when I try to install it, it says not enough disk space. I claims 6.5 gig... and I have way more than that available.

Help. Me. Please.