Very Computer

I have windows 2000 professional - all has worked well
until this week.  there is an administrator and 5 users. 4
of the users can no longer access internet explorer or use
the "my computer" function or access "control pannel".  
the error message that comes up is "runtime error" and it
shows the path to explorer.exe.  It also says "this
application has requested the runtime to terminate in an
unusual way. Please contact the support team for more
information".  The administrator and one user can still
access these functions with no problem.  All users are
classified as "power users".  I have the the latest
updates for Norton Antivirus and I have run a complete
scan of the hard disk. I don't know how to fix this