After installing XCOM r11.6 for Windows and starting the XCOM GUI with a Non-Administrator user you may encounter message: WARNING: Could not open/create prefs root node Software\JavaSoft\Prefs at root 5. To resolve/circumvent this you can:
1) Invoke XCOM GUI with a user with Adminstrative privileges, then onwards
non admin users can run the GUI without warning being displayed.
OR
2) Edit Windows Registry as follows -
Create "Prefs" key and provide full access rights to all XCOM users.
Here is the Registry path -
For XCOM R11.6 32 bit
---------------------
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\JavaSoft
For XCOM R11.6 64 bit
--------------------
HKEY_LOCAL_MACHINE\SOFTWARE\JavaSoft