There are already four primary partitions on the drive, which is why you can't create another. You'll have to sacrifice one of the partitions to do what you want.

There are two hidden partitions - one small Dell diag partition and a larger Media Direct partition. The other partition is the Vista restore image.

You can boot the Vista CD, delete the existing two non-hidden partitions (the C: drive and the restore drive) and create two partitions as you wish.

The Dell diagnostic partition should not be deleted - you may need it someday. It's too small to be useful for anything else. While you could remove the media direct partition, if you do the media buttons will no longer function.