Don't Turn Off That Computer!

It's common to find security advice to turn off your computer when you're not using it. The idea is that the more you are connected to the Internet, the more time attackers have to work on your system. (Hey, even they’ve said it.)
This is turning out to be bad advice, at least from a security standpoint. In fact, by shutting down your computer for hours at a time you are increasing the vulnerability of the system at the time you turn it back on. The longer it is off, the more vulnerable it will be when it comes back on.
Why? Because when the system is off your security software cannot update itself. When you turn your system back on, if there is a relatively new attack underway, the odds are reasonably good that your computer will be attacked before it has a chance to update itself. On the other hand, anti-virus companies and Microsoft usually have fixes available for vulnerabilities before you are likely to encounter them.
So, ironically, you're better off, from a security standpoint, leaving the system on, as long as your security update subscriptions are up to date. This is a separate issue from whether you believe that it's better for computers, from a wear and tear standpoint, to be on all the time or turned off periodically, and certainly separate from the environmental and power cost issues.