You've guessed it!
Your most likely problem is lack of sufficient drive space. Both Windows and PP use huge temporary files to do their thing. Also, by the nature of the way harddrives work, they need a little more "turn around" room when updating large files. You know, like that old c&w song, "Give Me Forty Acres to Turn This Rig Around"? OK, maybe that's too old..
For more details on how I'd configure harddrives, etc., check out this post..
http://www.videomaker.com/forums/viewtopic.php?t=4681