Memory pb with Vista

Hi, guys!

I recently install Vista Ultimate on one PC (HP Pavilion a1700n).

I remove the 40 GB IDE HD from the HP PC and put it into a Compaq EVO 510 PC.

Result: Vista does not boot.

I have searched the subject on the web but still have not found a solution. And the Microsoft Vista so called “step by step” help seems awfully complex to me.

Has anyone a simple way to solve this problem?

I have also tried the reverse (from compaq to HP PC): Same result!...

With XP I could boot into safe mode or just boot normally and XP would detect and install the new hardware it finds.

However, with Vista I am stymied!

Any (simple) practical solution?