FUCK YOU! You're worried about only having 299GB?
On my linux partition(which is the one I use most commonly) I have a max of 10GB, with 4.4GB left. My windows doesn't boot, there is an issue with my bootloader which I haven't gotten around to fixing.
Go to add or remove programs, and just have it order entries via file size in descending order. \win. I doubt it's a virus.
O'Yea, like mina said, because the filesystem used by windows is SHIT, it fragments overtime, you should run a defragmenting operation after mass file deletion\installation(not that it will create space though.) With that much harddrive space though, I doubt it's fragmented at all.