My C drive is constantly being filled up by something

[jjshabado]

Having no knowledge of OPs computer knowledge my guess would be that you're not emptying the recycling bin.

At least, if my parents called me up and said exactly what the OP said, I'd be 99% sure that was the problem. If you actually checked the space on your hard drive after deleting and there was free space, then obviously this isn't the answer.