If you have four primary partitions then it doesn't really matter how much space is unallocated. You won't be able to create any more partitions on that drive.
Boot Linux from a LiveCD and post the output of
With a standard MBR partition scheme you're only allowed to have four partitions. That's either four primary partitions or three primary partitions and one extended partition. The extended partition can be divided into as many logical partitions as you want.