Hub

In a hub, a frame is passed along or "broadcast" to every one of its ports. It doesn't matter that the frame is only destined for one port. The hub has no way

of distinguishing which port a frame should be sent to. Passing it along to every port ensures that it will reach its intended destination. This places a lot of traffic on the network and can lead to poor network response times.
Additionally, a 10/100Mbps hub must share its bandwidth with each and every one of its ports. So when only one PC is broadcasting, it will have access to the maximum available bandwidth. If, however, multiple PCs are broadcasting, then that bandwidth will need to be divided among all of those systems, which will degrade performance. 

For easy of use, both the FastEthernet card and such Dual-Speed hubs are usually configured
for automatic configuration : They are supposed to detect the type of hub or network card and
then select automatically the proper connection speed.

That works in most installations very well, but (like usual) NOT always:
sometimes the hub and the network card do not work properly together and the automatic
configuration fails: the network does not work (as it can be tested via the TCP/IPcommand).

As a first diagnostics, have a look at the indicators on your hub: