By network, I am assuming you are talking about a router/hub. It will be more difficult to get it going on her computer considering she is using Windows 98 SE.
First you are going to want to install the software (that came with router/hub) on her computer. Then configure the router/hub. In most cases you can do that using Internet Explorer, and connecting to 192.168.1.1.
I cannot give you detailed instructions without knowing which router/hub you are using. It also may be difficult connecting a USB modem to one, without the proper conversions.