Hi,
Bootcamp works beautifully with the X1900XT, I spent all of yesterday working in XP.
The apple installer didn't install the graphics drivers properly itself, so here's what I did to make it work.
go to system in control panel, click hardware, click device manager
click display adaptors, it will probably have an exclamation point (but may not)
right click the device under display adaptors and select properties
click driver, click update driver
click no, next
click install from a list or location, next
click include this location... and select the apple driver. This is in c:\program files\apple mac drivers (or something like that, I'm doing this from memory)
click next and it should work.
the card definitely works 100% (and beautifully) with xp so keep trying.
Another option is go to program files/apple mac drivers, etc and then the ati folder, (there are 2, use the second one)
click setup and let that do its thing.
or get the latest drivers from the ati website like Wintrmte suggested.
Good luck!