http://en.allexperts.com/q/Windows-XP-3282/disabling-program-installation-XP.htm said:All you need to do is add the user that your brother is logging in with to the Restricted Users. To do this, click on the Start button and select Control Panel. Open up the User Accounts utility. On the Users tab of the dialog box that pops up, double-click on the user that your brother logs in with. On the Group Membership tab of the dialog box that pops up, select the Restricted Users option. Click Apply and OK. Click OK again and you are finished.
You will want to make sure that the user you use to log into the machine is still a member of the Administrators group. If you and your brother use the same account, then you should password protect your account and then add a new user account, which is placed in the Restricted Users, for your brother to use. If you need help adding a user, let me know.