People here are neglecting for the most part to note the difference between consumer and corporate lines.
Typically, corporate/business machines are built a little differently and your requirements of them are different. Consumer machines are, n all honestly cheaper grade machines a lot of the time, and they change configurations and models more often. Corporate machines are intended for ease of administration and reliability.
For home machines, I'm a strong believer in Dell. Their support has fallen down a notch recently for my corporate machines though, and we've dealt with some truly incompetant salespeople in their accessories and server sales groups. (sending the wrong drive, no rails despite requesting the right rail and drive kit, and repeatedly dropping the ball at every chance they could get.). Our earlier puchases from them were spectacularly well handled, but their service department was either all promoted, moved, or outsourced. Talk about shooting themselves in the foot - they've made us so disgruntled that we're reluctant to continue buying from them until they fix their problems, and while we're not a huge contract, we did buy their higher end servers in multiples).
Gateway and Micron both make pretty decent home machines as well. Every Micron I've seen and used has lasted nigh unto forever and has been well designed in and out. Gateway uses more standard parts than HP or Compaq's home lines, so they'd be my third option. They are having financial difficulties and losing marketshare, so you might want to be cautious.
For corporate machines, (and this is from an educational heavy use lab perspective), Compaq machines have performed admirably. Their service department might not always be fast since you usually deal with the local reseller, but they have the parts, and they do support for 3 years. We also have very few hardware failures - the main failures we see are keyboards (students pounding on them day in and day out), and floppies (students sticking various mistreated floppies into them, forcing them in/out, etc). The failures we do see are mostly attributable to normal or, in the case of some students, abnormal wear and tear.
Additionally, Compaq provided one of our major requirements: being able to build the *same* machine for longer than a few months. We prefer that an entire lab be composed of the same system specs, and we sometimes have to upgrade them a dozen or so at a time rather than 80+ machines at a time. Compaq was able to continue to build the same machines for almost two years, including late replacements for dead systems, etc. That was a very, very important thing from an administrative viewpoint (imaging, system maintenance, etc).
Compaq makes good servers too, although I've been speccing Dells due to the price delta - that runs about 25-80% last time I priced them. HP servers suffered the same price delta, and I'd chose Compaq servers over HP.
HP I have little personal experience with beyond their home systems. Those...are evil. I've had more issues with driver support and lack of upgrade options on Brios and their ilk than I've had with Emachines!
Oh, and while I'm mentioning Emachines- they're actually pretty darn nice machines for the price. Pretty standard components, reasonable expansion, and they're cheap enough that you can buy one of them and replace it every year or two for the price you'd pay for a Compaq consumer grade of similar specs with a three year life cycle. We use an Emachine at my apartment as our primary web and ftp server, simply because it's little, cheap, and works.
Z.
(to back up my claims of expertise: I'm the lab administrator for Eastern Michigan University's Cybercrime and Police/Fire Staff & Command schools, as well as network administrator/computer services at one of the largest digital post production houses in the Midwest.)