Use Driveremap.exe from Citrix on your “virgin” Server 2003 installation and you can remap the C: to M: (default remap)
Caution: Do not use the driveremap utility on a server with .NET 2.0 installed. You can install .NET 2.0 after you remap server drives.
If the remap selection is greyed out:
http://support.citrix.com/forums/thread.jspa?forumID=1&threadID=35950&tstart=0
[...] popular public links >> remap Remapping your C: drive on Windows Server 2003 Saved by nickelbackrox13 on Tue 07-10-2008 [...]