UAC problems

Hello,
I need to connect to these 2 pcs:
PC A and PC B

I run the same QS on both PCs from non-administrator users.

Once connected on both PCs, I run an application as an administrator it happens that:
- PC A show the window where i can put user and password
- PC B the desktop is no longer accessible

Both have these common configuration:
- WIN7x64
- UAC on notification always
- local user is not an administrator

What is this different behavior due to?

thanks

framag