You can edit boot.ini file, so as to remove the option to boot to XP that was in C:\ Drive.
1] Right-click My Computer, and then click
Properties.
2] On the Advanced tab, click Settings under
Startup and Recovery.
3] Under
System Startup, click
Edit.
Generally, the Boot.ini looks like this (in your case):-
[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(2)\WINDOWS
[operating systems]
multi(0)disk(1)rdisk(0)partition(1)\WINDOWS="Windows XP Professional" /fastdetect
multi(0)disk(0)rdisk(0)partition(2)\WINNT="Windows XP Professional" /fastdetect
Here remove the line which points to
partition (1) that is C:\ Drive.
In the
default field, make
partition (2) so that it points to D:\ Drive.