I'll tell you what you can do IF you do the research on it.
I've had a emachines that was given to me several years ago.
It was actually during a time when I couldn't afford to build a new custom desktop because I had to spend money on a new roof for my house.
That emachines T6420 was better than my custom 2004 desktop at the time.
Just last year the motherboard failed on it.Instead of buying a replacement motherboard I tried something different which worked.First of all I didn't want to lose the software and other applications on that machines hard drive as I had also unfortunately purchased a few DRM title games for it.I did salvage a lot of the data though off the hard drive.
What I did instead was purchase another similar chipset motherboard with the exact same network interface as the model (MS-7207) used by the T6420.Also it had the same Integrated graphics chip in it.emachines used a slightly modified MSI 7207 Socket 939 CPU type motherboard for OEM name brand computer manufacturers.I picked a Biostar motherboard that had a different socket which was an AM2+ socket.
Also it's capable of running Athlon II's and Phenom II's( X2's,X3's or X4's) up to 95 watts TDP maximum on the CPU.The original T6420 ran a socket 939 Athlon 64 3400+ CPU.Anyway I had a spare AM2 type Athlon Le-1600 CPU and some spare DDR2-800 RAM.I replaced the dead motherboard with this Biostar model.
Initially I tried to keep the configuration as close to what the emachines given to me initially had (1 GB RAM,put in no discrete graphics card,kept the same dial up modem card in it etc).
On starting with the old hard drive with Windows XP on it I did have to reactivate Windows (probably because I tried it before on another machine and at that time it asked to be activated (which I didn't do at the time)).
In other words it might not have been necessary for me to reactivate Windows had I not tried it in another machine.Although connected up to a broadband network the reactivation somehow only would work on the dial up modem through Microsoft's servers.It took me 4 attempts but eventually worked (perhaps the dial upmodem was not working quite right (old) or Microsoft's servers were busy).
Well it passed Windows Product Activation with a different but very similar type motherboard in it except again it is a different socket motherboard.
It could be quite possible without again reactivating this to upgrade my CPU type with a much more powerful and more modern CPU.
I have once before replaced a dead motherboard on a custom desktop using this method (same chip set (or very similar),same built in Network chip type but no on board graphics chip).
So again it's possible to replace the motherboard to a more modern type which is capable of running newer type CPU's.However the motherboard has to be very close in having the same chipset (or similar),exact same Network chip and same on board IGP or no on board IGP.To pass Windows Product Activation one must keep the system as original as possible.Once succesful then one might be able to upgrade the CPU.
See here http://www.aumha.org/win5/a/wpa.php
There are a few issues and questions in that link that are nebulous at best.
1. does one need the same exact N.I. chip and MAC address to get all 3 votes to pass or could one get a couple of votes if the N.I. chip is the same but the MAC address is different?
2.On CPU type can the Processor Type be the same or an enhanced model with a higher speed?
Anyway I didn't think that my revamped rebuilt emachines would pass with the other board but it did.I may try to upgrade the CPU perhaps after the end of this year because I will upgrade a CPU in another machine and use the spare Athlon II 240 on this rebuilt machine.
By the way now I do have 4 newer custom desktops (3 quad core AMD systems and that other system with the Athlon X2 240 in it.)