Emulating games on a Windows OS is actually very easy. There are plenty of programs for emulating everything from GB to the PS2.
Generally you have the software and either a rom image or a disk (image or otherwise). Emulation on the simpler platforms like the GB, GBC, GBA, SNES or NES will not require additional tweaking beyond simply running the software and selecting the image.
More complicated systems such as the Dreamcast, PlayStation and PlayStation 2 will require that the emulator be set up with plugins; these specify how different qualities are rendered on your system (Graphics, Sound, Controllers, etc.).
Check out these sites for emulation:
The Emulator Zone
== Products ==
Microsoft Virtual PC 2004