Adobe Flash Update Wont Install - I FIXED IT!

I have a number of VirtualBox VM's running Windows on a Windows 7 Host.
Regularly when I come to do an Adobe Flash update for one of the Browsers (more often than not it happens to be FireFox) and the stub download occurs ok and you fire off the exe, avoid all the crapware and it gets part way through the download and install and fails ... nearly every time.

I discovered two things:

1. I was saving the download to a shared drive running off the host machine and mapped into my VM's ... quite a few installers will not work across the virtualised drive mapping!!!!
   ... including Adobe Flash.
   Copy the downloaded installer (e.g. flashplayer21_xa_install.exe) to the system drive of the VM and 95% of the time it will install without a hitch.
2. The other 5%, I wound up closing the browser as well, as soon as I kicked-off the exe

 99.9999% Flash update success rate now !  :-)