It used to be that when an OS was hung, and you can't control+alt+delete, you could at least power the computer off. Nowadays, the hardwire switch is connected to the operating system, so you can press the "off" button all you like, it's still hung. You have two choices, really: you can change the BIOS to accept a hard reboot after holding down the power switch for five seconds, or just unplug the damn thing from the wall.
The latter does not work with laptops (with working batteries), and in some cases, the former. I have a Win2000 laptop from work that hard freezes a lot, and I have to disconnect the power, then unhook the battery to get it to reboot. Holding down power switch for hours wouldn't reboot it.
Pain in my ass. Linux never does this to me. Up yours, Microsoft.