Sounds like your hands are tied a little here. However, Windows Operating Systems absolutely hate running out of hard-drive space, especially on the system drive. You will have continuous, unexpected problems unless you free up a lot of space.
Note that you won't be able to defrag the system drive until you have at least 1GB of free space (preferably at least 20% free, so really you want 2GB free). When a drive gets 100% full, bits of files are stored all over the place (to cram them in) so general performance plummets.
I suggest you carefully clean out/move all the files you can. There is loads of advice on the web to help. Have you, for example, moved the Swap Files to another partition? Deleted all the $NTUninstall folders (note, you won't be able to uninstall a Microsoft patch once you have done this)? Searched for DMP files? As mentioned above, look for TXT and LOG files, and TMP files. How about CHM files?
Normally, if you are careful, it is easy to reclaim a surprising amount of space.