How do I do a fresh winXPpro install on an hp?

I reformated the hard drive on a hp pavilion 7940, after removing it from the computer. I installed winXPpro and it works fine – until i put it back in the hp machine. then it don’t even boot up. when i put the hd in a generic ibm compatible itworks like a charm. Is there a special code I need to type in at the time of installation for the hp machine to read the op sys?