I'm trying to partition drive C but it won't let me.

New laptop is Toshiba Satellite and had the store configure it for me since I'm new to Vista.

1. Through Disk Management, I shrink drive C and now have an unallocated space of 51G.
2. From the unallocated space, I create a new simple volume. It goes through the Wizard, but then I get the following error message:

"The attempted operation is invalid. Either the parameters specified are invalid or the operation cannot be completed on the selected object."

3. The contour of the unallocated space is highlighted in black instead of green. Is that an indication of something? And shouldn't it say: Free space instead of unallocated?

Additional info:

There are currently 4 partitions. They are:
1: 1.46GB Healthy (EISA Configure)
2. C: 121GB NTFS Healthy (System, Boot, Page File, etc.)
3. D: 5.88 BG NTFS Healthy (Primary Partition) - This is reserved for a recovery
4. 5.34 GB Healthy (Primary Partition) - Don't know what this is for

My 51 GB Unallocated is between C: and D:

I'm logged in as an Administrator. Are there addition privileges that I would need to have? UAC is off.

Thank you in advance.