Not recommended as it entails too much fiddling, and there's a risk you may not be able to log on to Windows after a restart.
Presumably you have only one hard disk now which has 2 partitions; it's booting into Windows fine, but drive letters are shown as F and G.
You cannot use Disk Management to change either the system partition or boot partition.
To do it manually, you need to (1) change the path to the userinit.exe file in the current XP registry (to point to the new drive letter); (2) change the drive letter F to C manually in XP registry or use 98 floppy to do an fdisk /mbr command; (3) if the above succeeds, and you manage to log on to Windows after changing the drive letter, you need to change all previous references of drive letter F to drive letter C in the registry and Windows shortcuts (such as program shortcuts).
(1) and (2) are the risky part; (3) is the tedious part.