Odd. Wonder if there is a bug in the registry entries for the game. Still telling windows its there but it isnt.
Have you emptied your recycle bin then defragmented?
Maybe, just maybe, doing so will show windows its not really there and force it to recalculate.
And just to be sure, your system is clean of any spyware and viruses?
I must admit, I too am stumped. The file search should have showed any large files.
That's why I'm thinking registry. But I'm not gonna send you in there to check.

Registry modification isn't to be taken lightly.