Hi All,

Can the user login/access the machine during a maintenance window and if yes how do i prevent them from logging in at that time? Can this be done from SCCM itself? Also, by user logging in, can the update result in a failure? Thank you. 

yes they can access the PC during a MW. There isn't anything that you can really do to prevent them.

Generally the SU will NOT fail if a user logon during a MW, however they will get prompted to reboot, if one is necessary.