There are four general Categories of Network Problems.
1. Physical environment
2. Electrical problems
The physical environment has everything to do with the function of a network and for that matter all the things it is connected to. Normally if you feel comfortable in a room with a computer, the environment is probably OK for the network and computer equipment also. The primary areas of concern in any environment are temperature, air quality and magnetism.
Electrical problems happen quickly and often without warning. In order to prevent problems you need to plan for the installation and maintenance of the network and the equipment that goes with it. Be sure everything has proper ground and adequate wiring.
A computer virus is a destructive program that is written to alter files or the system configuration and then copy itself onto external disks or other computers. The main damage is that viruses alter files and configurations or grow exponentially, thus interrupting or slowing down data flow.
To protect against these aggressive enemies, you need to have the best Anti-Virus program you can afford. You can either have a program that is installed on each separate computer or a one that is installed on the server. It is far more economical to have one that runs on a server and it is far easier to administrate. You don't want users to have to update or maintain an Anti-Virus program because it can be time consuming and may cause them to spend time on something that is not their job.
Most networks have some sort of security to prevent unauthorized access and to control the overall system. The Network administrator is responsible to control what is shared and who has access to what. Most users don't even know they are denied access to something until they try to use it.
Passwords are normally used to provide security. However, periodically the user forgets the password or simply doesn't type it properly. Normally a network will be locked out after a certain amount of tries. The system administrator is the only person that will be able to correct the problem by either assigning a new password or totally resetting the account.
The above problems are only the main problems. Other things can happen to a network. However, if you pay attention to these you should prevent most catastrophes. Proper planning will prevent most major problems.