Right click on a drive and select change drive letter.
You will need to change whatever c drive is first. Change it too Z, Y,K.... something like that (just to get it out of the way for now). Then you can change your Hard drive to C , then change the other drive to whatever letter you like......
Can't change the drive you are booted to in Disk Management. Won't let you.
Even if you could, there are very specific registry settings that reference the boot paritition (system root) by drive letter. XP won't boot unless every one of these are changed, too.
For some reason, XP looks at ZIP and JAZ drives as removeable hard drives and assigns them the C: designation. Unplugging the ZIP drive and doing a clean install of XP is the only way that I know of to correct this. Once XP is installed. Plug the ZIP back in.