Also see: Emulator
"Bochs is a highly portable open source IA-32 (x86) PC emulator written in C++, that runs on most popular platforms. It includes emulation of the Intel x86 CPU, common I/O devices, and a custom BIOS. Currently, Bochs can be compiled to emulate a 386, 486 or Pentium CPU. Bochs is capable of running most Operating Systems inside the emulation including Linux, Windows 95, DOS, and recently NT 4. Bochs was written by Kevin Lawton and is currently maintained by this project. [...]"