your Network circuitry can handle gigabit speeds (1000mbps)
but it says 100mbps in your notification area because that is the speed your router/modem is capable of.
The network circuitry is used for both network and internetwork transfers; If you have a router then this routes internet traffic to the proper target machine on your network, and each PC can communicate with one another.
the maximum speed of a network transfer is the lowest speed of the network cards of each PC as well as the router itself. in my case, for example, my desktop is capable of 1000mbps, and my wireless laptop is capable of 300mbps. My router is capable of gigabit speeds. in this case network transfers between my laptop and my desktop will be limited by the laptop, which can vary (since it's wireless).
between my two gigabit enabled desktops speeds can approach 500mb/s for transfers between the two. However this is only for direct transfers of data; this speed is of course limited by hard disk speeds during transfers of files/folders between shared drives.