The options are usually in F8. When you start the computer and the BIOS displays its POST, (Power On Self Test), messages then just before Windows itself begins to load, start tapping the F8 key gently, once a second until the Safe Mode list of options display.
If you miss the timing and Windows begins to load, wait until you get to the Welcome screen where you select which user or enter your password, then open the icon bottom right of the screen to select Restart and try again. Or if you don't see the welcome screen, wait for Windows to fully load before logging off and shutting down/restarting.
It might help if you said what this black screen says.
You may need the Windows 7 Setup DVD to perform repairs.
Every so often I get the black screen of death saying "missing operating system". I read that I need to do the starup repairs that might fix the problem but I have no clue how to get to this startup repair place. Can anyone help me?