HP - (from my personal experience) AsusTek makes their motherboards and VGA cards in the Pavilions. So the Quality and stability should be the same as Retail, but HP puts a different BIOS chip than retail and AsusTek Provides no support.
DELL - is a good IC manufacturer. I really do not know who makes their motherboards, but usually it is pretty good. But usually there is limited BIOS feature's. And I had 2 Dell Dimensions (p2 @450) I found in a dumpster after a bank was upgrading and it works still. Hehe if that isnt quality I dunno what is.
Compaq - The WORST of them all. From my experience LIMITED CPU/RAM/VGA upgrade. Bad quality, usually just dies after a year and a half. Limited BIOS also. (Gateway is in the same Category as Compaq for me)
BUUUT!! I LOVE those propietory cases, they look better that that Aluminum stuff people pay over 200 bucks for. Stainless steel and heavy components make a better case rather than cheap, flimsy soda cans.