If you use Windows, here is some scary information for you. Wired reports on a study that shows an unprotected Windows computer becomes infected with some kind of malignant virus or malware just 20 minutes after being connected to a broadband connection (e.g. cable modem or DSL).
Wired wryly notes that that is not even enough time to download a typical set of patches from Microsoft.
I get asked all the time why I use Macs. One answer (I have several reasons why) is that I have not had a virus on any Mac I've used since 1993. Yes, there are fewer viruses written for Macs, and that plays some part, but the Unix operating system is both more resistant to infection and has much stronger controls than Windows. And out of the box, the default settings on a Mac are designed to make the machine very resistant to attack.
I still get asked all the time about compatibility. It's never been a problem for me. Microsoft Office on the Mac is virtually identical to the Windows version, and I open, read, and edit Office documents created on Windows on a daily basis. Bottom line--I get more work done with less effort and less cost.