Raid problem

I have been trying to set up Raid on a Windows XP system with 2 hard drives, both are Seagate, 120G & 200G. The operating system is on the 120G drive. I got through all of the setup & when I try to boot with the Raid enabled in the bios I get the error message, “NTLDR is missing, press any key to restart.” This just happens over & over until I go back into the bios & disable the Raid option. I was trying to set it up using mirroring. I’m new to Raid, what could I be doing wrong?