Yes you can use the power button to turn off your computer now.
In the past it was not recommended you use the power button to turn off your PC as Windows would not go thru its shutdown process and save all information and you ran risk of losing information in files you may have just worked on.
However with newer motherboards (the main circuit board in the PC) in the 2000s (ATX) the power switch when pushed sends signal to Windows to do the shutdown process.
However you should normally just click the power button not hold in, as a click starts the shutdown process and holding the power button down for about 10 seconds arbitrarily shuts the power down. If your PC will not shut down normal then holding the button for 10 seconds is way to arbitrarily turn off the electricity to it.
With Windows 8 there is no longer a Start button with the option to shutdown and using the power button is a suggested method.