Not required. Simply ensure that there is only 1 admin account and you have the password. Make their accounts limited. Windows set to auto-update by any user. avast! and most other AV anyway auto update under any user.

I have followed this policy for several years with my family and have not had to ‘clean’ any of the systems at all. Once in a while when I visit, I login as admin and update the other items like java, etc.

On my Win7 system UAC is at maximum with password prompt.