If you want to run Windows on your Mac, you have had up until now four main choices.
Now there is a fifth choice, Virtualbox from Sun.
VirtualBox is a family of powerful x86 virtualization products for enterprise as well as home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers, it is also the only professional solution that is freely available as Open Source Software under the terms of the GNU General Public License (GPL). See “About VirtualBox” for an introduction.
Presently, VirtualBox runs on Windows, Linux, Macintosh and OpenSolaris hosts and supports a large number of guest operating systems including but not limited to Windows (NT 4.0, 2000, XP, Server 2003, Vista), DOS/Windows 3.x, Linux (2.4 and 2.6), and OpenBSD.
I have downloaded it, but not yet installed it or tried to run Windows on it. Certainly looks like it could be useful for a whole range of virtualization solutions.
So what are the other four choices of running Windows on a Mac?
Boot Camp
This is an either or situation, which means you are running Windows or OS X, therefore if you want to change applications you need to reboot.
Has the advantage that you are when running Windows for all intents purposes your mac is a Windows PC. This means that you get the full power of your Mac for running Windows.
You need a licensed copy of Windows XP *SP2* or Windows Vista
Comes as part of Leopard, so if you have Tiger you will need to upgrade.
Parallels and VMFusion
http://www.parallels.com/en/products/desktop/
http://www.vmware.com/products/fusion/
These applications create virtual Windows PCs, which allow you to run Windows (and Windows applications) at the same time as OS X and therefore you can switch between them withoout needing to reboot. There is a slight performance hit when running Windows in this way but for most people this is negligible. Also you will need a lot of RAM to run these, at leat 1GB and preferably 2GB of RAM.
Coherence mode on Parallels allows you to run Windows applications seamlessly alongside Mac applications.
Both require a Windows licence, but doesn’t need to be SP2, with Parallels you can even install and run Windows 3.1! Parallels XP support is excellent, however support for Vista is less good, so you can’t use things like Aero for example.
Crossover
http://www.codeweavers.com/products/cxmac/
Allows you to run Windows applications (some but not all) within OS X and you don’t need a Windows licence as you don’t install Windows.
Downside is that it doesn’t work with all applications.
Can I use OEM versions of Windows on my Mac?
Generally the answer is yes. Though if you purchased a computer with an OEM version of Windows then the license only covers that hardware and according to the terms of use can not be transferred to another computer.
These OEM versions will work with Boot Camp and/or Parallels or VMFusion. Though as OEM versions you won’t get any support from Microsoft, for that you will need to purchase the full retail versions.
Note that Windows Vista Home can not be used with Parallels or VMFusion due it licensing restrictions.
Summary
Therefore there are now five choices when it comes to running Windows on a Mac. Which is best? Well that depends much more on your needs when it comes to running Windows.