Hi Aiglon,
What Operating System are you using?
For Windows XP Home or Pro it should be as simple as allowing the computer to boot up, and then going to Programs, Accessories, System Tools and then running System Restore.
Find the date (day before you let your son use it) and restore to this point.
If its Windows '98, you may have to go into Programs, Startup and find the offending toolbar application and delete it (unless it is part of Internet Explorer?)
If it is '98 or ME, it may be that you have a utility called System Snapshot which allows you to restore to a particular point, however my memory is a little hazy on that and I don't have a machine here running '98 or ME.
If it is part of IE then you should be able to switch it off from TOOLS and then INTERNET OPTIONS and then via the ADVANCED TAB? have a look for the offending toolbar and de select the check box. You may have to restart IE before it dissapears.
Hope this helps.
F - Wyg