If he is a Limited-class user, he should not be able to install software (easy enough to confirm that, try downloading and installing a program, perhaps
FilZip or WinZip, while logged onto his account). You may also want to grab the
SafeDisc patch so games on SafeDisc-protected CDs can run under Limited accounts.
To enforce the use of the right account, obviously you should password-protect all accounts, and also enable Fast User Switching. Another good idea: go to Start > Run and run gpedit.msc to open Group Policy Editor. Now drill down to
User Configuration > Administrative Templates > Control Panel > Display and enable Screensaver, then enable Password-Protect Screensaver and enable the lockdown timer at some sensible setting, maybe 10 to 15 minutes. This way, if you happen to leave an Admin-class account logged on, at least it'll lock before too long and he'll be at the Welcome screen where he can log onto his Limited account.
edit: you don't need to specify any particular screensaver executable for that to work.
My last suggestion is to make yourself a Limited-class account too, and use it for your day-to-day usage, so that even if he manages to sneak onto
your account while your back's turned, he's still not empowered to install software.
Hope that helps
