boot problems

I’m useing win/2000 dual boot with xp-pro on asus motherboard, I had 512 ram added 256 more and win/2000 won’t boot, xp has taken over, it will only boot to xp. I have 21 gig on win 2000 7 gig for xp, just testing.I need to boot to C: drive.I Bootedup with bootdisk and it wont load.