Dell Inspiron 1525 F11 and Down arrow keys don't work after reinstall OS

Recently purchased non functioning Dell Inspiron 1525. Message was needed new m/b. When started would run for a few seconds only then crash. Checked RAM, HDD and peripherals all OK. Replaced m/b, formatted HDD and reloaded OS (Vista) after reinstalling Mediadirect 3.5. Starts and runs perfectly but the down arrow key wont work! Have tried all the suggestions on forums etc., No go. The keyboard works perfectly on another 1525 I have restored. Did a system check and discovered the F11 key also is non functioning. Any ideas?