It ought to be F8.
When you say you get the boot menu, do you mean the menu for dual boot, or BIOS Setup? You may be pressing it at the wrong time.
I think you should press F8 when the black and white progress bar first appears (although I'm not sure). Just keep tapping it, until you see the Windows Advanced Options Menu.
The menu contains a lot of boot options, one of which is Safe Mode.