I have commented in a couple of threads about people wanting to have a Mac computer and a Windows based computer so they don't feel that they are missing out. I did fail to mention an option in both threads so I would like to start over with a new thread. You do not need to have separate physical machines. You do not need the most expensive machines. You do need some basic computer user skills and a little time and curiosity!
The first option is one I have mentioned previously and one I use most often, is virtual machines. There are a couple of brands of software that you can use, VMWare Fusion and Parallels. I use Parallels, I find it works better. I run this on my MacPro (Snow leopard), I allocate resources for the different VMs (virtual machines) that I will run, the setup/ install will walk you through this. I currently run Windows XP, Vista, Windows 7, Ubuntu, and Ubuntu Server as separate machines on my computer and I can run some or all simultaneously.
The next option is to boot directly into your "additional" computer. This is called dual boot and you run your OSes natively, that is you install your retail version of the OS of choice and follow the setup/ install walk through for partitioning and installation. There are a couple of choices of software to do this, Bootcamp (preinstalled on Macs and the easiest to use) and EasyBSD. Some people like this option better as it is very simple to do. Your computer, upon booting will give you the option of which OS you choose to use, then you can log off and switch between the OSes as you like. Another reason you may choose this route is because you do not have the resources to share out (CPU, RAM...) and/ or the tasks that need to be done are resource hogs, requiring the full use of your CPU, RAM, video/ graphics cards... Most of my resources are used on the Mac side and I use my Windows machines for proprietary/ legacy software and technical support. Remember,with this option you will use one OS at a time.
This may seem expensive and/ or technically difficult, but I assure that it is significantly cheaper than buying and maintaining new computers and it easy to set up and customize. There may a few open source projects out now that you can utilize and the resources I mentioned are available retail and on the internet. You have options so don't be afraid to try, you can have your cake and eat it too! Good Luck and have fun!