Yes you can. Here is the trick:
1. Insert a Windows 7 DVD into the Superdrive. ( It can be downloaded from http://technet.microsoft.com/en-us/evalcenter/cc442495 or elsewhere and burned to disc.
2. Start Boot Camp Assistant
3. When it comes time to reboot and install "Windows 7" at the part of reboot where the mac makes a "chime" sound, press and hold down option key.
4. Wait a while, then you should see your harddrive and soon a cd with "Windows" will appear.
5. Use the arrows keys only to choose the windows CD then press eject key to release the disc. The Windows 7 disc will eject.
6. Insert the Windows XP with SP3 disk, wait for the cd with "Windows" to reappear then press enter then, install XP like you normally would with Boot Camp.
7. Insert the Snow Leopard disc or Leopard disk after the Windows install is complete to install Boot camp drivers.
8. Run Apple Software Updates under Windows to get the latest released drivers from Apple.
9. Run Windows Updates a few times to get all the latest MS Security Software and Patches atc.
This works under Snow Leopard, Lion and Mountain Lion