do not believe that XP allows one to have 2 differently calibrated monitors on one system.
I have XP with 2 differently calibratet monitors and it works fine with my ATI Radeon, but there's something wrong, usually with nVidia drivers, and many times I couldn't properly calibrate two monitor setups of my friends and clients - no matter if the system was XP or Vista, any additional profile managing software (Color Control Panel Applet and so on) also didn't help .
The problem was that the system identifies both monitors as the same one, and associetes the same profile to both devices as a result. For the first time I thought, that maybe the graphics card has only one LUT, but it wasn't the case - I could manually load different linearisations to both LUT-s using PM Calibration Tester, but no matter how hard I tried, I couldn't associete two different profiles to two different monitors. It's especially irritating when you work with wide and normal gamut display, and the CMM is using wide gamut profile while managing colors on normal gamut display, and desaturates colors.
If someone knows the solution of that issue, I'd be really grateful to learn it. My observation is that it usually doesn't work with nVidia cards, and it sometimes work with ATI Radeon cards, but there's no rule.