On my ASUS board, the F8 key is used to get a Boot Devices menu and switches to Safe Boot options after the Post and before Windows starts.
Using F5 during the Post is supposed to get into the Safe Mode options.
But the machines are so fast now, hitting the key at the correct time is difficult. If you have Fast Boot enabled, it might be even harder.
Mike's suggestion of checking the Safe Boot option on the boot tab in msconfig.exe might be the easier way. While in Safe Mode, uncheck the box to boot back into regular mode.