If TOAD shortcut is set to open "Minimized" in Windows and a database connection is set to connect automatically, then Toad will give a 'Cannot focus or disable an invisible window' error when launching.
Set Toad to not open as a normal window.
Set Toad to not auto-connect to a database when launching.
This issue is fixed in Toad for Oracle 188.8.131.52. This version can be downloaded at http://questsupportlink.quest.com/prod_upgrade_alert.asp.