windows xp - how can I get more free space from one partition to the other with a Core 2 duo E7500 2.93 GHz

My C drive has 10 gb and my H drive has 500. The C drive has very low free-space so I can not defrag properly. Always around 7-10% free-space. When storing data or software I use the H drive because it has lots of GBs but some memory always shows up on C drive. I think C drive is the primary drive. I would like to transfer some of the unused memory or free-space from H drive to C drive or make H drive the primary drive. I have 3.21 GB of Ram