Turn it off, or password protect the screen saver, and password the BIOS.

Buy any "Surfer Protection" software is a very short term solution. Needs to be constantly updated, and/or it'll become intrusive, and/or he'll figure a way to beat it.

Since you have a DSL connection, you can turn that off. If you have a router (I have a Linksys) then used it's firewall and disable all traffic during the time he's there, or only outgoing traffic if you need to constantly download your email. Disabling the traffic works wonders for me. My daughters were spending all night on the internet, so I programmed it to shut down at 22:00. If there's a need, I can open it, and in severe cases, shut it down earlier.

No need to spend any money.