Perhaps it was a Pagefile that was eating up HD space. Usually it's set to match your RAM, but it can be tweaked higher or lower. Personally, I disabled mine since I never put my PC into sleep mode and, from my understanding, that's mainly what it's for since the partial power-down would otherwise wipe your active memory.
Randomly, I wish I could find out what eats my memory from time to time. Win7 lists 5 types in the manager (Free, In Use, Modified, Standby, Hardware) and the Modified memory steadily creeps up over time to where anywhere from 4 to 40 days after a boot I'll start getting low memory messages despite having few programs open or active. For example, right now I have 1845mb in use, 4762mb sucked up by modified, 1453mb in standby with 128mb free. Uptime's nearing 18 days. Checking processes shows nothing out of the ordinary, matching more along with the in use. I've tried process trimmers and memory cleaners to see if they'd help, but no go.
Violence good. **** bad. Yay America.