The system administrator is responsible for following things:
LAN and Server administration
Sound knowledge of server configurations
Sound knowledge of Windows Server/Active Directory/Group Policy/Role Definition
User administration (setup and maintaining account)
Verify that peripherals are working properly
Quickly arrange repair for hardware in occasion of hardware failure
Monitor system performance
Create file systems
Create a backup and recover policy
Monitor network communication
Update system as soon as new version of OS and application software comes out
Implement the policies for the use of the computer system and network
Setup security policies for users.
A sysadmin must have a strong grasp of computer security (e.g. firewalls and intrusion detection systems).