Hitting the F8 key at the right moment while your PC is starting up is much more difficult on fast modern hardware. With boot times as low as a few seconds, the window hit the F8 key and bring up the safe boot menu is very small. Practically impossible for most users unless you have the reflexes of a ninja.
In the event that you need to enter Safe Mode to troubleshoot a problem there is an easier way. Just open up the System Configuration utility by running msconfig at the Start Menu or Start Screen. Then check the Safe boot box and select the mode you want.