If you are using the XP install on the 2nd drive to use these forums, etc, then that XP sees it's own drive as C and any other HDD as D, E, etc. That's by design as Windows works best when it considers it's drive as the primary drive.

So, you could format the D drive, but here's the problem. When you installed XP onto the 2nd drive you setup a boot.ini on the C drive, (the 1st drive that the BIOS sees as the Primary drive), to enable dual boot. Dual boot needs to know which OS is where, and stores that in the boot.ini file on the first bootable drive. If you format that drive, (either using some system disk floppy for the C drive, or from within the workable XP and pointing to D), you will delete that boot.ini and that will make your workable XP install unbootable.

That's where my knowledge ends. I could suggest swapping the drives around so that what is now D becomes C, the bootable drive, then reinstall XP on it, then format the old C drive from within Windows. But my knowledge there is pretty slim and I would guess you need to change jumpers, or so on.

May be others here will know more.