I can't install Perimeter because the installer tells me that I don't have enough hd space. I have about 300g of free space. Says something about there being not enough space in the destination partition. Kinda strange since I've had no problems installing any other games lately.

Theres a bug with old installers - though perimiter is new enough that I'd expect the developers to have avoided it - where free drive space is stored in a 32 bit variable. Such a variable can only hold numbers up to 4Gb, with the end result being that the installer only sees the free space above the nearest multiple of 4Gb. I you have 8.1Gb, it only sees the .1Gb .
The only fix is to adjust your free space (delete something, make a duplicate of a big file or something along those lines) so that you have sufficient space above the 4Gb multiple.

Thanks.

Did you ever find your answer?

Well, yeah. I moved some free space from D drive to C drive (both are on one physical drive). And I also think that I removed those temp files somehow, but I can't remember how I actually did it :)