I've read through (I think) most of the replies/solutions on this thread. Some had me excited that a potential fix had been found, only to be disappointed in later replies that the same individual reported the "fix" didn't hold up - shucks!
In the end I followed the advice to simply replace the mobo and be done with it.
I ordered the mobo: ASUS Micro ATX DDR3 2000 AMD AM3+ Motherboard M5A78L-M/USB3
The Asus board is almost an exact drop-in replacement for the m2n78-la. The form factor is identicle, the processor from the old board drops right in, and the USB headers will also plug right in - connecting the front and back USB connectors.
The only change I had to make was to re-arrange the header plug which contains the power-on switch and HDD LED connections. If you look at the owners manual for the ASUS board pinout and compare this to the pinout of old board you'll see the differences.
Please note however, that you will (most likely) have to reinstall the OS. This however is not a big deal if you can borrow someone's install disk. I used my son's Win7 disk and used the exisiting Windows license ID imprinted on the OEM sticker on the outside of HP chassis.