Clarification.....
My understanding is that the original OS won't install on any other mobo than an EMachine.
Emachines have "restore discs" and a "recovery partition", Windows is present on the recovery discs, but is only usable (I believe) with the specific model number Emachine it came with.
Should you try to use a standard Windows disc, you will get an "administrative password required" error, due to the eprom in the board. On the upside, even with a full reinstall of Windows, called a "Destructive Restore" you don't have to reactivate. So, thank God for small favors I suppose.
I checked out Ebay, but none of them have Nvidia chipsets. I thought I read somewhere I could use a motherboard from any manufacturer, but it had to have the same chipset as the original mobo to allow me to reuse the original OS? I'm trying to go the cheapest route possible so I would prefer to reuse as much parts as I can.
Not even close to true, see above. Even a direct model replacement won't work. Windows is independent of the chipset being used, or you couldn't install a copy on any machine. What I think you mean is that a HDD with Windows installed stands a better chance of operating correctly without being reinstalled, if it is transferred to the same model # board. Even at that, XP may ask you to reactivate due to different serial numbers in the board and LAN.
As to reusing the older components, that would force you to backward engineer the system to use use possibly obsolete components. The HDD (if SATA), the optical drive(s) , maybe the memory,and definitely the case are pretty much all that wouldn't hamstring you into a retro machine. The T2825 is getting long in the tooth, with DDR memory, probably a PATA HDD, and no capability for a PCIE X16 Video card. It does have an AGP expansion slot, but these cards are getting a bit harder to find nowadays, and don't perform as well as PCI express.
I'm a little retro myself so, a bright spot here is that Newegg still has stock left on Windows XP (OEM) and, they have it in all three flavors.