centos 4 installed on P4 won't boot on P3

[admin at homemachine.net (Tom)]

Hi,

I installed centos 4 on a P4, configured it (and it booted fine here) 
then sent it to the DC to be put in a P3 but it won't boot, displays 
GRUB and halts.

The DC guys tried grub-install in rescue mode but still the same.

Any clues? Is there some difference between P3 and P4?

thanks
tom