Really sorry,i should have explained you properly.
Go to start and type msconfig
Now select '' normal startup and reboot''
Does it work now? If no then
Do you have something called ACER LAUNCH MANAGER ?
This is needed for your function keys to work properly.
Extract run the setup file,Install and reboot,does your function key work now?
If not download this
Open the extracted folders until you can see the setup.exe file.
Right click on the setup.exe file a choose properties.
Under Properties choose Compatibility tab
Now you can see an option '' run this program in compatibility mode
Check mark it and select windows vista service pack 2
Click ok and run the setup file and restart.Let us know if it works