if you got the same mobo as the dead one, then the original windows install along with the rest of your files can stay the way they are (you would not need another XP disk or a "re-do").
so you have a few options:
1. pay BB or another replair service a ridiculously high amount of money to replace a garbage emachine mobo, with another garbage eMachine mobo.
2. buy a better mobo* than the current one (you can get any micro-ATX board)
2a. install it yourself or have a friend install it for you
2b. have a repair service install it for you (you will have to pay them labor, but you'll avoid the retail markup they'll charge you for the board itself)
*=you will need a retail copy of windows. the good thing is that a retail copy will not be limited to just one machine, and it will be a "clean" copy without all the bundled crap that comes with boxed-PC recovery disks.
3. buy a new PC (you'll only need a tower because you already have a monitor, keyboard, etc.)
I believe the best option would be to simply replace the mobo with a better one. expect to pay about $40-$50 for a mobo, and then about $95 for WindowsXP (home editon w/ SP2). all together you'll have better hardware and a clean OS.
whatever you do, don't goto BB. they want to charge you $300 for a garbage $20 motherboard, and then another $200 for less than 1 hour worth of work???