Issue with Avira Free Guard n limited user

[d2fanatic007]

So I love Avira, I use it almost everywhere but I cant seem to solve this issue on 1 of the pcs in my home. When I go out at night my brothers friends, they are in their late 20s n swear they know computers but apparently not since I usually get bothered for a fix. They this pc when im not home and its on a Windows Prof SP-3. They only have access to a Friend account which is a Limited User. But they seem to find a way to turn off the avguard off and when I come back all hell is arisen n if you try to start it from the icon its grayed out.

I logged into my admin account n had to start it via services, which it should have automatically since its listed Auto for the Guard service. What are they doing to disable the guard and how can I stop that. Cause they keep adding an addon for Firefox called Bestshopping or something. I just dont want them to be able to turn off the avguard again. This is his pc but I have admin rights tryin to keep it up 2 par but cant seem to cause of 1 of them knowing how to bypass the guard even in Limited User.

I have atm uninstalled Avira and installed CAV (Comodo Antivirus) into the Firewall which I use as standalone, cause they cant disable the Comodo guard w/o the password and I have suppressed the warnings n such so they cant allow stuff. I would really like to use Avira again since CAV is so new and has alot of FPs n dont need it cause Windows issues.

Any help would be great, n some kinda explanation of how they got the guard service off n grayed out of the tray icon from a Limited Account

[mnosteele52]

As far as I know you can't stop Avira from being shutdown under a limited account, you have to purchase the Pro version for that. The Pro version you can password protect any aspect of the program.