Windows XP.

 

To start Windows XP into safe mode, here's what you need to do:

 

While the computer is booting up, press and hold down the "F8" key on your keyboard. Doing this should bring up the "Windows Advanced Options Menu". From the Windows advanced options menu, use your up and down arrow keys on your keyboard to select the "Safe Mode" option, press "Enter" and windows XP will boot into safe mode.

 

Windows Vista.

 

With Windows Vista, booting Windows in safe mode is done using the same procedure as is used to boot Windows XP into safe mode:

 

Just press the F8 key on your keyboard while your computer is booting and then use your up and down arrow keys to select "Safe Mode". Hit "Enter" and Windows should boot into safe mode.