What type of internet connection do you have? And what is the Internet connection attached to?
I think your best bet for dealing with the Internet connection is a small router. Most routers also have DHCP capabilities which will help out a little bit for all of the computers connecting.
This means all computers on the network will take the following information automatically.
IP Address
Subnet Mask
Default Gateway
DNS Server
This sorts out the network connection for your users.
Now to making the systems able to talk to each other. For Windows XP you shouldn't have to do anything else for them to see each other. For Windows 98 you may have to install the NetBEUI protocol - this is a broadcast protocol and will allow the other machines to see them by name.
Then its simply a case of creating the appropriate shares (for printers and files). For the scanner I'm not sure you can share that between all users since it probably only plugs into one PC.
The only issue you will bump into at some point is user permissions. Since Windows XP requires user authentication your users will have to enter usernames and passwords to enter the shares unless the username and password match those the user is currently logged in with.