old post now, i am surprised no support help here????
to solve low disk space problem in server, if you do not want to use third party partition tool, you have to rebuild your server for a large OS partition. I guess you do not like to do it.
Diskpart.exe cannot help in this issue, as contiguous space is required. so considering nothing is perfect, third party may be a good choice here. Certainly, do not forget back up all crucial files.
see here for how to extend system partition: http://www.windows-server-partition.com/resize-raid-5-partition-windows-server-2003-r2.html