You will have to install the Raid drivers for XP when you boot for the new disks to be seen. Start the PC with the Windows CD in the drive (And set to CD as first boot device) and then when XP starts to load you will see a message "Press F6 to install third party Raid drivers" or similar. Hit F6 and it will continue to load and then ask if you want to install the drivers. OK and away you go - XP will load onto the Raid array.
What I should also say is that you will first need to get your serial ATA drivers onto a floppy disk and have this inserted into drive A when you hit F6. These SATA drivers will be on your motherboard CD.