LR4.

I installed LR4 on my laptop with Windows 8 and it works fine.  I then installed it on my desktop using Windows Vista.  It opened fine and I used it once.  Shut it down and I have not been able to open it since.  So, I uninstalled it and re-installed it and the same thing happened.  It opened  the first time and worked fine, and then would not open again.  Any ideas?......Help!  Thanks...Mick