There are far more qualified contibutors here than me but here's my opinion.
Memory and hard disc space are different. 6gig is small for a hard disk these days but useable. There is software which will manage partitions but I don't know if it can sort out existing partitions, ie ones that it has not created in the first place. You could reformat and partition the disc using fdisk into a single partition (and wipe everything that is currently on there) and then re-install XP.
There are a couple of MS support articles here:
http://support.microsoft.com/?kbid=313348
http://support.microsoft.com/kb/q255867/