highly doubt it
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.