Cannot sync

I keep getting this message after I compile the Blink program.
The LED is lit on the board. (Both of them).
avrdude: stk500_getsync(): not in sync: resp=0x00

How do I fix this?
I am using Windows7.
Thanks, Ken

I found the solution just now quite by accident.
I have three Uno R3 boards.
Two of them were from a few years back and have been in a box. The third is a new one that I just ordered for a course I am putting together.
The two older ones are not working.
I finally opened the package and tried the new board and it worked.
It looks like the other two boards are not functioning. I get the green power light and a blinking yellow light when I reset them with the button, but the Blink program will just not work on them.
Hope this helps someone who may have the same problem.
Ken