Lock down your computer (
Google search of a few guides that might help), create a limited account for him (
another Google search), and take karate (I'll let you search for that on your own) so you can deter him from doing that in the future.
If he's determined to be malicious, he probably already has a key stroke logger installed, and probably already knows your password. But don't change it right off, get rid of the keylogger first.
http://www.google.com/search?hl=en&s...an&btnG=Search
Also, learn what processes are supposed to be running on a computer so you can pull up the Task Manager (Ctrl+Alt+Del and Task Manager, or right-click on the bar at the bottom of the screen where the open programs usually show up, or use Ctrl+Shift+Esc) and stop any processes that shouldn't be running.
I'm also moving this to the Windows forum (as you said you were learning VB, I'm assuming you are on Windows), where you should be able to get more ideas.