Assumption you have the Network Analysis Extension for ArcGIS (9.3) Sp3: Your 'Network'
Creating a Network >Arcatalog
(you must understand the TYPE of network you are trying to build first!)
http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Creating_a_network_dataset
You can automate this with ModelBuilder (Python)
http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Add_Locations_(Network_Analyst)
{linear unit}
The search tolerance for locating the input features on the network. The parameter includes a value and units for the tolerance.
{source {snap type};source {snap type}...} point and line
Allows you to specify which sources in your network dataset will be searched when finding locations, as well as specifying what portions of geometry will be used.