trouble formating

I have first boot device set to CDrom, but still it doesn't boot the CD. I have done it multiple times on my own pc but now on my brother's it doesn't seem to work. damaged cdrom can't be the problem because it has no problems running any other cds. tried googling, but even on the tech help forums people couldn't determine the cause of this problem. the cd itself cannot be damaged either because I just tried it on my own pc and it runs the cd without difficulty. any ideas as to why it denies to boot the windows xp cd?