You must have heard about VMWare or Virtual PC; now, I will introduce you to Bochs. This is an open-source PC emulator written in C++. It can host different types of platforms based on different architectures. It is released under GPL license.
This PC Emulator, not unlike others, emulates an Intel X86 computer, common I/O devices and BIOS. Bochs can be compiled for the emulation of a 386, 486, or Pentium CPU, either X86 or 64, including additional instructions for MMX, SSEx and 3Dnow!.
It can be run in different operating systems, such as Linux, DOS, or Windows (Vista and Windows 7 included, according to my tests). This software is mostly used for keeping the host operating system clean when you need to use some unreliable (or incompatible) software: when the virtual operating system fails, the host doesn’t. The program also emulates every single hardware part of a computer, such as hard drives, cdrom, and floppy drives (Bochs even allows you to add hardware if the emulation is already running).
However, it also should be noted that the emulation performance is somewhat slower than in other emulators. In addition, unlike other emulators, Bochs provides a much less intuitive GUI. Still, if you only need to run old games that refuse to run in your current operating system, Bochs could be your choice.