XP I assume? I would suggest booting from the XP CDROM, and select 'Recovery Console'. Once in command line type bootcfg /rebuild (read about it
here) - or 'scan' first if you are nervous
- and try selecting the C drive as the system ?you want to use?. If that works you should be able to safely delete the Win installation on the D drive.
Depending on which system it has its little head into, you may need to copy Docs and settings from D to C to retain all the stuff you want.
If that does not work it may be that a
'repair' is needed. Remember that if you have not 'slipstreamed' your SP2 into a new boot CDROM you will have to reinstall SP2 again.
Happy Saturday
Over to the experts now.
Everything backed up first, of course