You need disc partitioning software:
I was lucky enough to get Partition Manager free on a magazine cover disc some time ago (one of the very rare occasions I’ve used a cover disc).
I have also use Power Quest Partition Magic, but this failed recently when I tried to partition a large drive (I don’t know why). The Paragon software “undid” the wrecking and successfully completed the partition.
There may be other free stuff; perhaps others could advise.
There is always the small risk that any partitioning software could "wreck" the disc, so make a backup of important stuff first. In your case, such a disaster would also mean you would, potentially, lose XP as well.