keeping user from accessing my computer on certain times

Archived from groups: microsoft.public.windowsxp.security_admin (More info?)

in the past few days, somebody accessed my computer and crashed it by going
into spyware filled sites. I would like to know how to block limited users
from accessing my computer while I am not around. my computer has two limited
and an admin (me). Is there any way to do this. Also, is there a utility
that lets me see what other users were doing while logged on and what sites
they went into (some type of tracker). I don't want to delete the accounts, i
just want to limit them further. One more thing, how do I restrict sites for
limited users. can anyone help me out.

who ever can help me out, thank you!

A concerned and angry administrator