How to merge to partitions of a hard Disk

Hello,

I am using window xp operating system. My hard disk has total capacity 74Gb. It consist total 4-partition, out of which three are having 19.5Gb each and rest is around 15.5Gb. I want to increase the space of my C-drive which is having space 19.5Gb. What i want to know that
1. How i can increase the capacity of C-drive.
2. can i merge any of the drive with the C-drive so the my problem get resolve. If yes please tell me how i can do it?