I run several windows systems here in my office as well, and for the most part they run well enough to be useful for a decent amount of time. As you say, a lot of it is just knowing not to open obvious junk Emails and knowing not to click on bogus web sites that ask you to download their software.
That having been said, the only machine in my office with problems right now is a 4yr old Dell desktop with WinXP on it. The video periodically scrambles and I have to reboot it, and about twice a week the whole thing blue-screens and dies. So far rebooting fixes everything so I haven't put any time in to fixing it yet. Neither of those problems are virus problems however. I think the blue screen is an overheating issue and I think the video problem is a video card problem. So both of these issues might be associated more with hardware (Dell) than with Microsoft.
One of the Macs in the office is over 7years old and the only problem I ever had with it was a burned out power supply, the repair of which resulted in the most incredibly perfect customer support call I've ever experienced. Here's what happened:
I came in one morning and the machine was completely dead. Obviously a power supply problem. So I called the Apple store in the Mall which is about 30 minutes from here. They told me to go online to their site and log an appointment with a "genius" before coming to the store. I was annoyed by that because I figured it was redundant and they wouldn't know who I was when I got there anyway. But I did it anyway. Then I drove to the mall with my desktop in tow.
When I walked into the store (a standard Apple store in a mall), one of the blue-shirts greeted me at the door and asked what I was there for. I told him and he looked me up on his handheld gadget. He found my name and my problem and escorted me to the proper place in the back of the store. The guy at the desk got to me within 5 minutes and confirmed that my power supply was the problem. I asked how long it would take to get a new one, expecting that he would tell me to come back in two weeks. But he said they had one out back and would have the machine ready in an hour. He then told me that there was a power supply recall on my machine from a few years back and he could apply that to my repair. I asked what that meant and he said it meant that everything was going to be FREE. He took my cell phone number and I went to get lunch at the food court. Almost exactly an hour later my cell phone beeped saying the machine was ready. So I retrieved my machine and sure enough, they charged me $zero$ for the entire repair. The entire experience was FLAWLESS. When the guy at the counter asked me for an evaluation of their service I told him it was almost *scary* good.
Then a month later Apple released IOS4.0 for the iPhone/iPod and screwed up everything. Now my iPod is clunky and I may need to get a more recent model to make it work again. It's hard to be perfect for long