I'm not sure how to break the RAID apart as separate drives. But, you should be able to install XP to the same pair of drives even if they are in RAID. To do this you just need to create a floppy disk with the proper RAID drivers on it and then use this during the XP install (usually by pressing F6 when prompted at the beginning of the install).
Other than that, you might watch your monitor during boot to see if you see something that says 'Press ?? to enter ???'. You might be able to enter your RAID controller BIOS and turn off the RAID. This of course could be a tad risky.