I think that would either be four things:
-A problem with how much capacity you have set for your PC restore.
-You have your virtual memory set too high for how much space you have left (but you still need at least 2 GB set aside for gaming), or you have your virtual memory on a setting where your system will adjust as it needs it.
-You have some narly virus that keeps replicating itself and other files.
-You have some files in limewire or a torrenting program that keeps trying to finish downloaoding a file and it just fills up the space as it frees itself.