Does not running as an Admin make any practical difference?
Most windoze hacks involve running the code of the attacker's choice (usually achieved by overflowing some data buffer which is on the stack, so the buffer data contents gets executed as opcodes when the program returns from the function call) and such code is more than capable of elevating itself to any privilege level. Especially at the next reboot
On top of that, many programs do not run properly unless under Admin rights.
I have had to deal with the "kids' PC getting trashed" issue a number of times, and IMHO the best way is to have a dedicated kids' PC, make an image backup (Trueimage, etc) and every so often just restore the image. Sometimes one has to restore the image anyway because the PC is so badly trashed it won't boot. My son once trashed the PC by installing some FSX aircraft model. I have Kaspersky AV on it anyway, not that AV software stops trojans which seem to be well capable of disabling such software.