1) In enterprise manager open your server, go to Management, ricght clikc on SQL Server Agent, go to the Job System Tab,
at the bottom, uncheck "Only users with SysAdmin priveleges can execute CMDExec and Active Scripting job steps" the hit the Reset Proxy accuont button. Put in the account you want to use to run your command.
2) Make the step in the job you want to do. do you execution as "copy \server1\myshare\*.pst \server2\myshare\"
make sure the user you put in for the proxy account has permissions to the shares, etc...
that should do it.