Mac Emulation
For folks with recent Macs (with Intel processors) - you no longer need to emulate a PC, Macs now use the same processors as wintel machines.
If you only need to run one or two Wintel programs, first try Crossover Mac from Codeweavers. It manages to let you run many Wintel programs without having to buy Windows itself.
If you want to actually run Windows, you can install it with Apple's BootCamp, included in OS X 10.5 (Leopard). That let's you reboot your Mac into Windows. The only cost is a copy of Windows (OEM versions are under $200). We actually have one client who found that an 8-core Mac Pro was the fastest Windows machine they could buy at the price, they use to for a rendering engine.
If you want to run Windows in another window while using your Mac (without rebooting) Parallels and VMWare Fusion both allow that, and each costs about $80. Parallels is more mature, Fusion seems slightly faster (but both are quite fast). Again, you also have to buy Windows itself.
If you are not sure if your Mac has an Intel processor or not, under the Apple menu select "About this Mac" and the processor info will be shown. If it includes the word "Intel" then all the above applies. If it says PowerPC, it may be time for an upgraded Mac!