How do I include unallocated disk space in the primary partition?
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.