For Windows XP

you open a terminal prompt by either going to start->all programs->accessories-> command prompt. or you can goto run and simply type "cmd". Once at the command prompt you type the following

"at" is the automated task
[time] - this is the time in which you which for the new command prompt to open, if the current time is 10:30 PM you would put 22:31. puting it in military time and one minute ahead(if you do not put it one minute ahead it will schedule the task to run the next day at the specified time)

/interactive (might be /interaction i forget to be honest) - this is sort of self explainatory
"cmd.exe" - this simply specifies which program or service you plan on running
once the minute has passed a new terminal prompt will open the prompt should be something liek "c:windowssystem32" now if you open up task manager you should see "cmd.exe" being ran as system. from here we kill the "explorer.exe" being ran by your username (or thje username your logged onto the system with) once explorer.exe is down, goto the new terminal prompt (with the prompt c:windowssystem32) and type "explorer.exe" that start menu will return.
now this will give you local admin rights on most machines but is not always sure fire like the thread maker said. unfortunately form the little testign i did with this, it would give you system rights on XP-pro machines, however on a domained network they best you could get was local adminastrative accts, change passwords and such