|mmm... agree re warranty etc. for the SATA.|
"If" you were to supply another c: drive (EIDE or SATA) then it would be no great issue to have XP on d: boot - much as now..
But if you do NOT supply another "c:" drive - as a Master to the current slaved XP drive... your options are limited...
Essentially (if no replacemnt c: drive) you run a repair installation to the current version (CD booot etc.) - and if it works... you end up with a version that now says it's on c: (not d: as now). All path statments will have been written afresh - fo the OS; not entirely sure what happens for installed apps etc. (Perhaps Wanderer can\will drop in here and advise\confirm in that regard?)
Otherwise you start afresh with a new installation to the current d: drive set as Master (as in the repair routine).
Drives are reasonably "Cheeeeeppp" these days (in some parts of the universe), so perhaps consider supplying another c: drive - either a warranty SATA replacement or maybe buy a standard EIDE model?