RESOLVED Very odd mkr driver question

Even I am stumped by this one.

I have two MKR's

One has not really had much use apart from mainly software testing

The other is brand new.

BOTH were working fine on win 7 x64 pro under various IDE's but currently 1.6.12 for testing purposes.

The older one (if you can really call it that) suddenly gives the exclamation mark in windows and wants a driver. If I unplug it and plug it back in it gives the new hardware sound and then the failed sound.

The newer one if I use the same USB cable happily does its thing and just works without demanding a driver.

Tried 3 cables and all the same on both boards one works the other does not.

Tried the slight wiggly of usb cable on both boards still same one works other wants driver.

Tried multiple usb ports / hubs etc.

Tried to install the correct driver (yes I know which one it is) and for the dodgy board it tells me

"Windows was unable to install your Arduino MKR1000"

"Windows could not find driver software for your device"

I can double tab and get it into bootloader and load up a sketch but the moment the sketch is in it returns to not having a driver.

Yes I know its very odd.

Does anyone have any ideas why one works like a charm and the other is recognised correctly but would want a driver ?

UPDATE...

It looks like a win 7 issue.

If I plug it into a server ( server2012 ) It works fine.

However that still leaves questions about why ONE of the TWO boards works whilst the other doesn't ?

Normally I am pretty darn good with computers and troubleshooting so if anyone has any clues at all or even a stab in the dark as to what could cause this then thank you in advance.

USBSER.SYS was replaced ta some point and identified as the probable cause.