What are you hoping to accomplish with this?
Is the motherboard broken, or do you just desire something better, to overclock with?
If it's broken, your best bet might be eBay, which commonly has consumer PC direct part replacements. This is the only guaranteed way to be able to swap it directly into the PC without having to rebuild the system, or repair Windows. I cannot for the life of me find an A8AE-LE in Asus' website, though, so the motherboard wasn't a retail product in the North American market. It looks like a Radeon Xpress 200 chipset, so another motherboard with the same chipset might get you good results, too.
If you're going to do a fresh build, anyway, any micro-ATX Socket 939 motherboard should do ya' just fine.
The problem, though, is finding Socket 939 motherboards to begin with these days. They're very much end-of-life. Anything you find will be a couple of years old, and you'll be limited to whatever oddball vendors happen to have them lying about in a warehouse. Newegg even has Socket-A motherboards in stock more often than they have 939s!
My suggestion to you would be to browse the Classified section here at OC.com - someone might be looking for a buyer for an older micro-ATX 939 motherboard. Plenty of folks sell regular ATX motherboards for socket 939 there, it seems... all you'd really need if you wanted to use one of these (which will be far more common than a micro-ATX board) is a cheap ATX case. Transfer all of the stuff from the Compaq, and you're the big winner. You could probably get a case for about $30, as you wouldn't need anything remarkable, and a good used board for maybe $50. Which is a bit cheaper than going $150 at minimum for a full platform change.
So, just so we all know, what model of Athlon is this, anyway? What sort of memory came with the machine, and how much of it?
ETA : If you go the full-ATX route, check this out:
http://www.gearxs.com/gearxs/product_info.php?products_id=10171 - $27 for a motherboard, NIB. I've never heard of or done business with GearXS.com, so buyer beware as always.
http://www.extremeoverclocking.com/reviews/motherboards/Foxconn_NF4UK8AA_1.html - Review. Not the greatest, but for $30, is it that important?
http://www.newegg.com/Product/Product.aspx?Item=N82E16811147103 - case, $30 shipped. Not pretty, not elegant, but it'll hold an ATX motherboard.
Remember, if you do something like this, be sure your PSU cables are long enough to reach in a normal case; some OEMs are really stingy with their cable lengths in micro-ATX cases!
