If you install a non-identical mainboard you'll most likely end up having to reinstall Windows indeed. Even if you find a similar mainboard, Windows might still require that you re-activate it.
I agree on the absurdly high price but that's what you have to pay for rare hardware of sometimes dubious origin. You could try some second hand websites other than Ebay.
Socket A is no longer being produced which is why you're having a hard time find. Personally, I find it a horrible platform that runs way too hot and I would suggest moving on to the new AMD sockets which are much easier to cool.
If you can't find an exact duplicate, you could look for mainboards that have the following characteristics:
Chipset Name NVIDIA nForce2
Chipset 'Northbridge', Revision/Stepping IGP, A03
Chipset 'Southbridge', Revision/Stepping MPC2, A04
If these are similar (You can ignore the stepping) there's a good chance Windows will boot up again.
The rest is optional but the more similar to your current mainboard, the more chances of you not having to re-activate:
http://h10025.www1.hp.com/ewfrf/wc/document?cc=us&dlc=en&docname=c00006476&lc=enAlso, this mainboard seems known as "explorer 2"
If you search for "explorer 2 hp" on Ebay, you get a lot more results.
