I installed PL/SQL Developer version 8 some time ago.
In those days I had windows admin right.
Now I don't have windows admin right.
Can I install PL/SQL Developer version 9?
Can I install PL/SQL Developer version 9 over PL/SQL Developer version 8?

If you have the privileges to add/change/remove files in the PL/SQL Developer installation directory, then it should work.