So if his dad happens to run a business using this (shared) PC and the messing around with system files, or subsequent virus attacks lose all his data, which he hasn't backed up properly for several weeks, 'cos he's been really busy lately, then you're willing to accept full responsibility for dishing out this kind of advice?

Also, if his dad runs the right kind of software on a daily basis, a keylogger will show up straight away, won't it?

My Firewall has blocked 20,947 attempted accesses in the past month and that's after I told it not to log the more routine kinds of events. Today's count is 429. I'm taking no chances.