One way to get round this is to go into the Control Panel, then User Accounts.
Set up a new account for your grandson and make it a Limited User, that way he cannot do things like install programs etc.
When he uses the computer get him to use that account and not yours (set up a password on yours to stop him getting in to it).
All your settings, and his, are held in separate places so if he makes any major changes on his ID you can still log on to your ID and use it.
The above instruction are how it works in XP, in Vista it may be different (but the principle still applies)