somewhere in administrative tools is "disk manaement system" or something like that - tells you tons about your HD. . . maybe you can find something about it in there?

ADDED: ooh!ooh! do you have quotas enabled maybe you hit your quota, OR your page file is on that drive, and once you come close to that amount of space it reports only what you can write to.

No, I didn't have to unmount anything to move the ISO I don't think, but I'm not quite sure I follow. My page file is at 192MB, I've never seen it higher, actually (max is at 384MB). I did clear out my history, but I hadn't surfed that much since the last time I cleared it out (about two weeks ago), but it must still be a big source of the data that's missing.

I think you are talking about Computer Management. It has a disk section for management (partitioning and formatting) and defrag. The disk management says that the partition is healthy. I don't have a quota set for any users.

when i mount iso's it has to dump to my page, but thats cuz i don't have much ram.

besides your page file isn't that big, so the iso unmount idea is totally bunk.