It's also free.

On my Mac in the company I use Parallels Desktop. Completely dissatisfied I am not, after initial teething problems (Installed Update was the program not start, support let me 2 days depend) my virtual Windows is running relatively smoothly. Only a twice within 8 months I had to set up a new virtual machine, because the old suddenly and irrevocably refused to serve.

Highlights: In particular, the fact that I use the Windows programs can assign files to the Mac desktop, so that they automatically open in the virtual environment by double, is very comfortable. Likewise, the fact that I can store Windows applications in the Dock. However, I must reject the win-software so that's still works so simple in irregular intervals the files.

Private I had bought Parallels Desktop MacOS 10.4. When I needed an update in order to continue using the software under MacOS 10.5 can, I wanted it should almost pay the first owner paid: This was (and remains) Parallel's Desktop for me completely died.

I now have to use since June 2008, the free software "Virtual Box" by Sun Microsystems in operation, under MacOS a virtual Windows XP Home Edition. And VirtualBox runs surprisingly well in the current version. And quickly.
And can, in principle, almost the same (except for the direct opening of files from the Finder): Here it is also possible to work in coherence mode, ie: Start Windows window directly in the Mac environment. You can set up a directory for file sharing between MacOS and Windows without any problems. And the clipboard works as bidirectional. Access to Airport's network and to USB device works without problems, and even sound could be activated easily.
And I can stop the virtual machine, save the current execution status and accurately continue to work on the spot where I left off.
Also: VirtualBox currently supports version 2.0 and 64-bit guest operating systems. That's not the case with Parallels Desktop. Parallels Desktop can also work with an established via Bootcamp guest - which can in turn VirtualBox currently not yet. I persönlilch feel not as a disadvantage, I've set up under VirtualBox my virtual Windows drive than 20GB large, to grow along with volume. Say: It will initially only as much disk space as necessary proves to specify the maximum size of 20 GB is reached.

My tip: first at least once to try the freeware. Who's like me absolutely sufficient here can save a lot of money.

Important: Windows XP has an integrated Service Pack 2 included. The virtual volume must be formatted when you install Windows definitely FAT. The virtual partition should be at least 10 GB (15 or 25 better) be large in Win XP, if other than the operating system nor additional software to be installed - eg. Office software, virus scanners, other browsers, etc.
And finally, as almost always applies: Low Memory (RAM) = little power, with a virtual XP 1 to 1.5 GB RAM max. basically completely sufficient.