installing xp after ubuntu7.04 and now can't reinstall ubuntu

[buro_1986]

i installed ubuntu 7.o4 live cd on hd1 making it ext3 partition and thereafter sometime i installed xp deleting that partition .when i tried to reinstall ubuntu 7.04 from my cd it dosn't boot,pc jumps to xp and the cd dosn't boot but other bootable cd works except linux pls help me and tell me what to do, i ll be very thankfull i tried other ubuntu ubuntu8.04 this cd also dosn't boot pls help me

[GreenN00b]

Enter bios setup when the computer starts up and make sure that CD is first in boot order ...