I allow my kids on it as I have already educated them as what is OK and which is not.
All private files are safely stashed away on a fully encrypted ext3 partition.
Nope, (CTRL + ALT + DELETE, Lock Computer) but everyone in our house have their own computers anyway (except mum, who uses dad's), my sister's is full of spyware/viruses, need to clean it infact!
No one touches my PC - partly coz of the above mentioned toolbars/spyware.