So I just built my new comp, it posted, etc. I got my linux LiveCD to work, shut it down, tried to install XP and it wouldnt work, gave me "Error Loading OS," tried several times with 2 working xp cds, no luck. Tried the Linux cd and now it gives me the same error.
Get something to totally wipe your hdd (zero write/fill) - back to nothing then try again.
Also check your boot order.