You may be able to save a little bit by limiting the size of your event viewer logs. These tend to be disk hogs and probably can be scaled down.
Also, look for log files (e.g. Web Servers etc. tend to write logs ad-infinitum). You need to make a call whether you can dump these (or some of them).
If you can figure out storing this (or something similar) somewhere:
JDiskReport
It is a useful tool for quickly telling you where the disk space is used, but does need a jre to run. You may be able to find similar that will run without java - google is your friend
Finally, have you looked at how much permanent swap file space is allocated? I'm not 100% sure that you can set it, but I would not be surprised - this could be eating a huge portion of your disk...
edit: also look to see if you have a memory.dmp file from your last system crash. This can be deleted, and (at least in XP and I'm going to assume in NT) you can minimise the size of this file when/if it is created.