how do you unpartition a hard drive?
A partitioned drive has one or more partitions on it. All drives have to be
partitioned (have at least one partition) to be used. Your question, as it
stands, makes no sense.
What is the current partition structure of the drive and what do you want to
end up with? If what you want to do is change from a multiple partition
structure to a single partition one, read on:
Unfortunately, no version of Windows before Vista provides any way of
changing the existing partition structure of the drive nondestructively. The
only way to do what you want is with third-party software. Partition Magic
is the best-known such program, but there
are freeware/shareware alternatives. One such program is BootIt Next
Generation. It's shareware, but comes with a free 30-day trial, so you
should be able to do what you want within that 30 days. I haven't used it
myself (because I've never needed to use *any* such program), but it comes
highly recommended by several other MVPs here.
Whatever software you use, make sure you have a good backup before
beginning. Although there's no reason to expect a problem, things *can* go
wrong.