How to extend the size of my C drive on XP

You must delete the other partition and consolidate them as free space just following the Windows partition. Under these conditions, you can enlarge the Widows partition to include the following unused space.

If the Disk Management of XP can't do it (try it first), some free utilities with good graphical interface are:

EASEUS Partition Master (Home Edition)
Paragon Partition Manager Express

I suggest to take good backups first, as resizing the C: system drive can cause Windows to become unbootable.

Try to use GParted within PartedMagic. It is a Linux based partitioning tool that can boot from CD, USB or network and do almost anything with partitions: resize, move, clone, etc. Works better than Symantec's PartitionMagic for me, and also the interface is much more nicer.

You could use the GParted LiveCD, it has an easy interface and a beginner friendly documentation.

Warning: Backup your data first (just in case), changing partitions can result in data loss, I mean 99% of the time it is working flawlessly, but the other 1%...gggrrrrrr. ;)