I've triad all of the windows access control etc, and the problem is that too many applications, (games mostly) assume that the user is Administrator.
I've done the same as the previous poster, installed Ubuntu, with Firefox, Thunderbird, Open Office etc my wife and children are fine.
The boys want to play some Windows games, so I've put VirtualBox on, loaded their version of XP which used to be in that computer. That lets me keep a virgin installation of XP with the basic games, so whenever they mess it up in future I can recover it in 5 mins - though they will lose their game saves.
My wife needs to run a .net application which I have not put on to VirtualBox, but I've no reason to think it won't work. Early days yet, but this seems to be working a treat.