Pretty sure the PC is 32 bit - will check later - it is 3 yrs old.
Also, just realised that the HP laptop was working fine - the sketch had a problem. For some reason, the Leonardo board cannot run the Tone() function - the sketch actually crashes so make sure Tone() is commented out if using the Leonardo.
Also, it can be very difficult to download a new sketch to a Leonardo. Pressing and holding the reset button until Uploading does help although it can still take ten goes to redownload to the Leonardo - watch the LEDs and when TX/Rx lights illuminate together for a second the sketch has probably downloaded. Note the Leonardo is superfast at downloading. I downloaded a Leonardo keyboard example sketch and it was very hard to re-download and the Leonardo was trying to overwrite the sketch with repeated characters which was worrying ( but what the sketch was programmed to do).
Here is a fix for the Tone issue: