C:\ is full

I have a W2K server (DC) with one basic disk partitioned with C: and D: volumes. The C: has only 23MB of free space left. D: has about 2 GB of free space. This is the only server on the network, and it is backed up with an SCSI tape drive. I have another HDD to add to the machine, but I don’t know the best way to increase free space on C: What is the best solution?