I assume this is a Windows 2000 server?
Is the disk a basic disk? If so, you should be able to create 4 partitions but this is the limit - is there a partition at the beginning called the system partition, some server setups (Compaq, Dell etc) create these to store tools and utilties usually taking up 30-50Mb.
If this is the case, I'd convert the disk to a dynamic disk which lets you create as many volumes as you like.
Cheers, Antony