I have around 25 gigs free out of a 220 gig hard drive. I've cut down on the number of programs I have running in the background, but it will still do it even when it's the only program running. I've even had task manager running to see if there was a spike or anything unusual in my system performance when it does stop, but nothing. I have no idea what is causing it.
Edit: I checked to see if my drive is fragmented, but it is not. 0% fragmented.