I've investigated this further. Just posting in case anyone else hits the same problem.
I keep all my B4A folders on a removable drive.
Somehow I developed a version of the problem described in this link
http://support.microsoft.com/kb/330137
I changed the drive letter of my drive.
Rebooted
Changed the drive letter back to what it was in the first place.
This sorted the problem.
It's obviously a known windows problem. The strange thing is it only seem to affect B4A, there was no problem getting word docs off the drive for example.
Thanks
PS I reinstalled B4A from a link I found in an old email.