I'm sure this is the classic response to these types of threads, but I'll go ahead anyways. Linux has come a long way.
In particular, Ubuntu has become as easy to use as a Windows box. I don't use Ubuntu, but I have installed it recently and I was really excited about what they are doing for the Linux Desktop. The install is easy breezy as they have literally removed all the scary stuff, in particular partitioning. It comes with a number of apps already installed, and getting new apps is as easy as opening the New Software app, picking what you want, and clicking install.
For those that can't live without their Micro$oft products, you always have the option of installing a virtual machine such as VMWare, VirtualBox, etc. I use VMWare when I'm forced to use a website that still insists on only supporting IE, though these sites are rapidly disappearing with the market share that both Firefox, Chrome, Safari, and Opera have secured (almost 40% combined as of Dec '09).
For you gamers...you're still screwed
It can be a scary step to take, but it is completely worth it. I haven't had to worried about viruses or malware in ten years. OK I'll get off my nerdbox now.