Get a dual-core Dell Vostro desktop with 19-inch widescreen LCD and Windows XP for $399 shipped
Dell's small-biz Vostro 200 comes in a slim case and sells for $399. It's a very complete system for the money, and a potentially good option for students.
Most $399 desktops (particularly those from Dell) saddle you with a crappy processor and insufficient RAM. Not so the Dell Vostro 200, which features a dual-core Pentium E2160 and 1GB of RAM. Granted, that wouldn't be enough memory to keep Windows Vista running smoothly, but this system comes with Windows XP Home (bonus!).
Despite the "Home" OS, this is actually a business-minded desktop. It includes a 160GB hard drive, a 19-inch widescreen LCD, and integrated Intel GMA 3100 graphics (no Call of Duty 4 for you). It doesn't come with a modem, speakers, or any preinstalled software. But you do get a 1-year warranty that includes next-business-day on-site service.
I particularly like Dell's space-saving slim tower, though don't expect much room on the inside for upgrades. Even so, this looks like a stellar deal for students, small-business owners, or anyone looking for a basic desktop on the cheap. The $399 price tag includes shipping.