Strangely, while trying to find the motherboard you mention online in the MSI support pages, they don't list that motherboard model, at all...
Looking at the ASUS, for the 5xx series chipsets, your processor never made it out of testing it appears with that board, it's stuck on a beta BIOS apparently?
Sadly, this means that the board may never actually support that particular CPU properly.
Windows XP can see multi-core processors just fine.
But make sure you have the absolute latest BIOS revision for your board... Which would be a BETA version, which means potential stability issues.
The last BIOS released was just over a year ago, version is 1804, I am surprised as that update enables AM3 processor support on your board.
Also, a motherboard doesn't need to know the name of a processor to work, the only thing a motherboard requests from the CPU is what voltage it wants to run at, the Multiplier, and the FSB that the CPU needs to operate at, and how many cores if applicable.
Can you get us a picture of CPU-Z as MoM asked earlier? I think something is disabling your core after reading this thread multiple times.