There are several things you can do. One of the easiest is to run the windows disk cleanup utility (I don't know how to launch it on all versions of windows, so you'll have to find that out). This will clean out the recycle bin, temporary files, and some other problem areas. Another likely culprit is the system restore utility. Again, I don't remember a step-by-step, but you might want to reduce the percentage of disk space used by this utility. As I remember, it defaults to 10%, which on large drives is a crazy amount of space.