I would suggest using an ubuntu liveCD and using Gparted to delete the C partition and extend the F partition, but partition magic will probably do the trick. Either way though after you extend the partition you will have to fix your master boot record. You do that by booting from your WinXP and choosing Repair, choosing the F: drive install and running "fixmbr" from the prompt. I have a feeling this will change the drive letter back to C, but I'm not sure. If not you can follow this tutorial to change it.
Change System Drive Letter in Windows XP