General Protection Fault. Serious error. Program was doing something to the computer that it shouldn't have, like write to memory occupied by something else.
Always a result of bad programming, either on the part of the program that caused the GPF, or another program running at the same time. Typically caused when some dink-programmer tries to bypass the operating system and access things directly; a serious problem in DOS (because it don't have the API required to control the hardware properly, and you HAVE to talk to the hardware directly) but not much in Windows 2000 (because the operating system has almost Gestapo-like control over the hardware and forbids most software from talking to it directly)