I have the exact same problem and have been looking for a solution. I do a lot of 3D rendering and my dual AMD 2400+ MP processors were working fine and all of the sudden they stopped working. I have them on a MSI K7D Master L running Win XP Pro with 1 GB of ECC Kingson memory and RAID 0 on 2 WD 250 GB SATA HDD.
What is the answer to this problem? I've been in a couple of other forums and they say one solution is to reformat, but I have to much info to go through that mission. I belive its a problem with Windows XP.
Please if you find a solution or if anyone knows the answer let us know.
solved the missing cpu problem. Have you been on windows update? make sure you do the amd driver update off windows update. Cured my problem. format is last resort in my oppinion. You could alwys install over the top and just re do ALL windows updates after and add service pack 1 if it isn't part of your xp install.
still haven't cured the crazy benchmarks yet tho. Hope this helps.
In short I think its a driver issue. i bet if you only see one processor in taskmanager.... and you look in device manager... you still see two processors. Have a check.