opening folders on file server is so slow!

I'm moderately clueless, and could use some help, please. Our small office network consists of 3 XP machines connected through a hub-type-thing (with blinking lights) to a Windows 98 machine. We connect to the Internet and our printers through this computer. Our "file server" is simply a shared folder on this machine containing lots of folders in which our organization saves everything. Opening folders on our server frequently takes a really, really long time. There seems to be no correlation between time of day, number of users, etc. etc. We all have shared folders on our own computers, and there're never any problems with opening things on each others computers. Do I need to do something besides "share" the server folder, ie change some network settings I don't know about?
Thanks.