Virtualization Tools
Some of the games above are old enough that an emulator/virtualised enviroment may be needed. Here are some of the most popular (and free) ones in order from specific to general use.
ScummVM - Allows you to run certain classic graphical point-and-click adventure games, provided you already have their data files. The clever part about this: ScummVM just replaces the executables shipped with the games, allowing you to play them on systems for which they were never designed!
DOSBox - Emulates an Intel x86 PC, complete with sound, graphics, mouse, joystick, modem, etc., necessary for running many old MS-DOS games that simply cannot be run on modern PCs and operating systems, such as Microsoft Windows XP, Windows Vista, Linux and FreeBSD.
Virtual PC - Lets you create separate virtual machines on your Windows desktop, each of which virtualizes the hardware of a complete physical computer. Use virtual machines to run operating systems such as MS-DOS, Windows, and OS/2.
Virtual PC Guy's Blog.