Many Operating Systems on one laptop

Not strictly CAD or 3D, but this page gives an overview of someone who installed several different Operating Systems on his laptop (a Mac Powerbook).

http://www.kernelthread.com/mac/vpc.

When you have a more recent computer, there is much better support for multiple systems, especially when your processor supports "virtualization".

You can use VMWare (or fusion for IntelMacs), Virtual PC (Windows only), Parallels (PC's and IntelMacs) and some Open Source solutions (qemu, PearPC etc).

I wouldn't advice using this for intensive 3D applications, but for many more common tasks it is usable. In the case of CAD software (e.g. running AutoCAD or Revit or 3ds max on a Mac, you are best to use Bootcamp from Apple to boot into Windows, to take full advantage of Hardware Acceleration of the graphics). Graphics Acceleration is still the bottleneck in many emulations or virtualization applications.