it's not really running at 100mbps, think about it in the first place your dsl provider gave you an advertised speed connection which is roughly about 768/140 (down/up) kbps to say the least. not even 1Mbps. i have a home network of 3 pc's my router detects 100Mbps too, but i highly doubt it.
you can go Control+Alt+Del and check your Networking and it'll tell you how much percentage of the slink speed you are using. and you do the math, and get the idea how fast it is.
100 MBS is the maximum transfer rate available, tipically the operating system do not fill all available bandwidth with a single connection in order to leave room to other connections (ie inbound and outbound LAN traffic with multiple machine simultaneously)
Ep, glad to see you come back and tidy up...did want to ask a one day favor, I want to enhance my resume , was hoping you could make me administrator for a day, if so, take me right off since I won't be here to do anything, and don't know the slightest about the board, but it would be nice putting "served administrator osnn", if can do, THANKS