Function keys not working on Dell Vostro running windows 7

I upgraded from Windows XP to Windows 7 this summer. I now need to use a secondary monitor that I use for some additional work I do this time of year. The function key (FN + F8) worked before the upgrade to switch between the laptop screen and the secondary monitor, but now I can't get it to work. It appears that none of the function keys are working. I tried to go to the screen resolution screen to detect the secondary display, but it won't even do that.

Any suggestions