I have the exact same problem.. i went from 1800 MP's
I have even done a clean re-installed windows xp with sp1 and then it detected the system as "MPS Multiprocessor PC" but still only 1 cpu i device manager.
and checked that there is not 1 graph for both cpu's
Previously the system with dual 1800 got detected as and ACPI multiprocessor.
I also went through and tried repair with F5 to force HAL selection but it does not show "ACPI multiprocessor".
Also tried this @ bottom , replacing hal loader fileshttp://forums.amd.com/lofiversion/index.php/t73601.html
but blue screened @ startup 000007b but following MS KB it just puts you in a loop and does not help..