The issue being that macs are just as likely to pick up a virus? Yes, ten years ago you were far less likely to pick up something nasty on a Mac, but crooks catch on to trends pretty quick.
It also won't run 90% of the software you'll want to use to run a business. Apple make great products but the majority of the business world is still standing by Windows. (Unless they're a fancy schmancy design company)
I'd recommend running Windows 7, I've been working with a couple of PC's...