Dell, hands down. I work at a major medical university and we use Dell desktops for the ENTIRE health system. I have an Inspiron 1100, 2.5 years old, fantastic. I bought the 4 year on site, accidental warranty coverage. Ok the phone service is a major PITA so I went to e-mailing my issues to them (have had the a/d adaptor replaced 4 times...bad design but every time it was shipped and here in 2 days with NO questions). I've had the mother board replaced due to a problem with the PCMCIA slot, probably my fault becasue my wireless network card sticks out and wasn't good to use it on my bed. Then my kids stepped on it and cracked the lcd. They sent an entire new lcd and the guy came to my work to replace it. That's the accidental coverage that's $79 and totally worth it. The 4 year is like almost $400 but how can you pass it up??? Just yesterday my kid knocked the a/c adapter in the dog's water bowl. I e-mailed them and a new one is on the way.
One of my co-workers bought an HP and it was a major piece of crap. He had to send it back to the factory 2 times for issues in less than a year. Once the warranty ran out, he was SOL. HP makes wonderful printers but their computers leave a lot to be desired.