HP's actually the best I've seen. Dell is BAD. Custom builds are awesome. :D
If you buy one of HP's new notebooks/netbooks, they have this thing built onto their motherboards that, lets say you want to add a 3G modem or bluetooth to an empty PCI-e slot, you put it in, OH WAIT, the BIOS isn't gonna let your PC boot because the PCI-e card isn't HP branded! And I wouldn't be surprised if they did the same with their desktop PC's now.
And there's nothing wrong with Dell, I used one from 2007-a few months ago, it worked just fine for me, never gave me any problems.
However, indeed, Custom Built PC's are the best.