The way Windows Setup works can be quite confusing... was the old disk with C: on it still IDE-0 when you installed XP on the other disk?
And you changed the Active partition setting from C: to D: yourself? [if you had tried to restart after that you should have gotten a "ntldr is missing" message and a blue screen].
Anyway... it reverted? Right...
=temporarily edit the boot.ini file in D:\ to look like this:

[boot loader]
timeout=6
default=multi(0)disk(0)rdisk(0)partition(2)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="Microsoft Windows Prof D:" /noexecute=optin /fastdetect
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows Professional" /noexecute=optin /fastdetect

-then during login you will be certain which boot.ini is being used- if it is this one then you will actually see the menu. The second line I added is a nonsense for your sys, something has to be there otherwise you will not see the menu; do NOT choose it, just let the menu timeout to default after 6 secs.
=Check, or set, D: to be Active, that ntldr and ntdetect.com [and boot.ini] are in D:\.
=Shut down and restart into BIOS, set to boot from cd and insert your XP cd, boot into the Recovery Console by typing R to Repair when given the choice during Setup. [If the sys manages to boot into Windows again accidentally then it may revert the settings above; redo them]
Let the loading of Recovery Console run to the point where it asks if you wish to start in E:\Windows, type 1 and Enter.
Now at the E:\WINDOWS prompt type..
fixboot D: -and Enter, and to the write query answer y and Enter again. Now type...
fixboot C: -Enter and answer y, Enter..... Type exit to quit, the sys should commence a restart.
=Restart into BIOS, set IDE-0 as boot device and let the system boot.
That should do the trick. Say what happens.... and if all is good edit boot.ini:

[boot loader]
timeout=6
default=multi(0)disk(0)rdisk(0)partition(2)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="MouMou's Windows Professional" /noexecute=optin /fastdetect

You can put whatever you like in there....
I for one don wanna be a cat... a couple of the things they do I find disturbing.