Eve, when you have a Compaq, Dell, Gateway, HP, etc., it really doesn't make much difference who the manufacturer is or what the model number is for the motherboard, the only place that you can get any info on them is from the vendor (Compaq, Dell,....). As Raven said these vendors use proprietary motherboards (i.e., a motherboard that is not available from anyone else even tho it is made by one of the major mobo manufacturers) and they generally are not very upgradeable. One reason they do this is so that you have to go to their website to find out things like what kind of memory modules they use and then they try to get you to order from them if you want to add memory (at a higher price than you can get the same memory module elsewhere).
One of the advantages to building your own computer is that you know exactly what components you have in the box🙂 BTW, anyone that can read and follow instructions can build their own computer, it is really pretty simple to do.