Press F8 repeatedly as the pc is starting up.
Early pcs used to go straight into safe mode, later ones give you options, if you get options use the up and down arrows to highlight safe mode and press enter.
It can take a while to get into safe mode as they sometimes hang on the screen full of text for a minute or two.
When you are in safe mode the screen will look terrible as its set at a very low resolution and colours as the graphics card drivers arent loaded.