We solved it this way:
In Windows 7 registry :
Change the value of ForegroundLockTimeOut in HKEY_CURRENT_USER\Control Panel\Desktop
from its default value (decimal 200 000) into the new value 0 (zero).
After that : restart the pc.
Let us know if this also works for you ...
There are a few threads about this problem in the actual forum about the "MS Office Addin", like these ones: