#1.
Admin CP ->
Modify User Groups -> click on
[edit] for whatever usergroup, scroll down and there is an option to disable private messaging.
#2. Requires coding, and I'm a bit too lazy at present.

Look around the forum, maybe you can find it.
#3. Install (or reinstall)
PhpTriad, insure you are actually running the apache console before launching your site, and make sure you have installed vBulletin properly with the MySQL database.