|The key varies with different PC's and OS's.|
It's usually either; F1, F2, F10, DEL or Escape.
When your PC boots, you should get a screen display. It will say something like "Press F2 to enter BIOS" and "Press F11 to enter Boot Setup"
Or something similar.
Whatever key it asks, keep tapping it till it loads. Or try all of the above keys till you get it right.
PEBMAC - The biggest cause of Computing problems.
Damn ID10T Errors.