UPDATE - I Hadn't realised that Microsoft had once again started to provide free downloads for all their browsers (they used to provide these a while back but stopped for some reason). The downloads are available here:
http://www.microsoft.com/download/en/details.aspx?id=11575
A friend of mine recently tweeted that their license for Parallels had run out. When it comes to running other operating systems on your machine most people opt for either Parallels or VM Ware. This process is often known as Virtualization.
Virtualization is not emulation, it's an actual machine running within your machine so you can be sure that everything will work as it should.
What a lot of people don't know however is that Oracle offer a free product called Virtualbox, which will run on WIndows, Mac Os and Linux. All you need is either a CD or disk image to create the machine. Virtualbox also supports virtual disks which means that you don't have to pre-allocate a large chunk of your valuable disk to your virtual OS, you can trick it into thinking it has 60GB of free space but only allocate it disk space as it needs it.
I mainly use it for making sure my sites are cross browser compatible although people often use it to run those programs that have not been ported to every operating system or to simply run a Windows version of Microsoft Office on a Mac.
*You need a CD or disk image of the operating system you need to install which may cost money