If you are a technophobe ... how do you get near to "technology" to use it?
anyone with access to a machine and a little knowledge can access anything on any machine ....
bottom line - the only way to stop it happening is
a - stop him using the machine
b - stop using it yourself.
webmail is less vulnerable ... a google account uses secure servers ... so as long as you don't "remember" your ID or password ... it's as good as you can get.
try using ccleaner
http://www.ccleaner.com/download
it's free ... and will do a better job quicker....