I'd actually look into
Acronis Disk Director Server for an "easy" fix. It works with RAID setups and can edit partition sizes. If you'd prefer a more "do-it-yourself" approach, I'd check these:
[ulist]
Virtual Memory, check the location of the pagefile. Move it off drive C to drive D. (I know the link is for XP, but its in the same place for Windows 2003) Set the initial and maximum sizes to be the same, 4096 (or 4095, can't recall what the actual maximum is). That way it won't fluctuate in size.[/*]

Make sure there isn't a hibernate file on the C: drive.[/*]

Try
clearing the Internet Explorer cache, including "offline files". Change the cache size to something like 50MB instead of the default.[/*]

Check for log files from other software and see if you can configure them to write to drive D:, then move the existing logs there.[/*]

You do not need to keep the unpacked service pack installation folders on drive C:. You can move them anywhere you like. I'd suggest if you really feel you need them on-hand, burn them to a CD.[/*]

Try
Windows Cleanup - use at your own risk. There is a demo mode that will show you how much space it will clean up and you can browse to see what it will delete. Use the demo mode first before running to make sure you aren't going to wipe anything important. If you think something there is important, then reconfigure and rerun demo.[/*][/ulist]There are many more things you can do, but without seeing it first hand I have little more to offer than this.