Replying to Computer help

ok, so here's my problem. I have 12mb left on drive C. But no, i dont suck with computers. I partitioned one of my drives so that 20gbs was for the OS and the rest was for all my files and what not. That way if i needed to reformat windows i wouldn't have to go about backing up tons of files. Ok well i just checked and since i started writing and its back up to like 200 mb left. Which is weird. I dont get why it's fluctuating like this. When i try to go and hunt down stuff to delete to clear up space i can't find anything. It says theres 9gb in documents and settings but when i look in the folders for it, the folders only add up to like 650mb or so. So where's all my space going? It gets really annoying cause it gets the computer running pretty sluggish even though im not using like any of the CPU or memory. Help?