Su.exe is a utility from the Resource kit in NT4 that lets you run a single program in a different security context than the logged on user. It is like the su command in unix. Suss -install will install the service. I am not sure but I think SU stands for Switch User. That is exactly what it does anyway.
For example,
Your user has the wrong system time. Also, the user has MS word, MS Excel, An Email program and some other applications open and claims he or she can not log off right now. If you are running this service, you could just go to a command promp and type:
su administrator
<Type password here when prompted>
time 12:00 (or whatever time it is)
then close the command window. Any command or application you launched from that window runs as local administrator so completes successfully even though the user logged on did not have authority to do so.
In Windows 2000 this is built in. If you hold down shift and right click on a program (Shortcut?), you will have an option for RunAs, which gives a nice GUI to switch users in.
DanielMc