I was looking here.
http://h10025.www1.hp.com/ewfrf/wc/documentSubCategory?tmp_rule=15025&tmp_task=prodinfoCategory&lc=en&dlc=en&cc=us&lang=en&
then here
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00757531&tmp_task=prodinfoCategory&lc=en&dlc=en&cc=us&lang=en&product=3339286&key=null&site=null#N124
and here
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00820191&tmp_task=prodinfoCategory&lc=en&dlc=en&cc=us&lang=en&product=3339286&key=null&site=null
It states that it is a Socket AM2 based motherboard A8M2N-LA (NodusM3)
using the GeForce 6150 LE chipset and a Marvell 88EC031 Network Chip.
I have before tricked a system using another socket type motherboard with the same chipset and network chip into working with Windows with/or without reactivating before on a hard drive.
See here.It was an emachines socket 939 and I used a alternative socket AM2+
replacement motherboard.
I think that the only reason that it required reactivation was that I used the hard drive on a different build before temporarily (for a few minutes) with a completely different chipsets,network etc just to see the machine working I shouldn't have done it though.
http://www.tomshardware.com/forum/54953-2-alternative-replacement-motherboard-emachines-t6420-other-models
It states that you can upgrade the CPU up to an Athlon X2 5000+ on your current motherboard.They are not being sold right now retail (sometimes however I have found older components in odd stores (like a Fred Meyer)).You might easily find one on ebay though (as many people upgrade their CPU's and resell their old ones).
If you investigate and find an alternative replacement AM2+ based available motherboard using the GeForce 6150 LE chipset and using the exact same Marvell 88EC031 Network Chip it's quite possible to reuse your older Hard Drive with Windows 7 on it and not have it reactivate or if it does require reactivation it might pass WPA as well as WGA.One could perhaps choose a much more powerful Athlon dual core family CPU like a Athlon II X2 265 as the replacement CPU instead.
Just some thoughts here if you want to investigate this further.However note that it can be quite difficult to look at various motherboard specifications and manuals to find one with the same network chip and chipset.
Very likely that it not be possible.
So far I have checked a few possible alternative boards for you and have had no luck finding one with the same network chip that's available at newegg.Again It would take a lot of time and investigation to do this.
You could however upgrade your CPU or plan on building another system (which can be quite inexpensive around $200 to $250 for a good motherboard,CPU and memory) and have the total hardware cost around $500 for a system far more capable than your current one.