Emachines are typically not proprietary hardware, Look on the board for the OEM's part number and let us know what it is.
Just use an xp cd and use their code, When (not if) you go to activate it, it will tell you that you need to activate it by phone. Call M$ up and tell them you changed the board and they will mate that copy of xp with the new system. There is nothing wrong with doing this, M$ hasn't really stopped alowing you to do this yet, But will soon. In press releases they say If you change the board in any system then you need to purchase a new licence, But they are not inforcing that yet as long as when they ask you how many computers is this copy being used in you say one.
Now the only problem you might have with their code is if the code is newer then the release of the cd, Then you just have to find a newer cd that's all.
Edit - I looked threw this thread and sorry if I missed it but what model emachines is it?