No, he probably wasn't wrong when he said 100Mbps; I wouldn't be at all surprised if the connection to the modem was with 100Mbit ethernet, in which case that's probably what the cable company will call it, regardless of what the speed actually is.
And just because a T1 is 1.5Mbps doesn't mean cable can't get faster. Case in point: I have Cox@Home, and I routinely get 3.2Mbps real-world speeds, even before you take network overhead into account. Speeds like that aren't at all unusual for DSL and cable providers, and I've seen plenty of deals for 10+ Mbps DSL in Japan and Korea, so I wouldn't be surprised if there are US cable companies offering similar theoretical maximum speeds, even if you never actually get near that.
Even though I'm getting way more than double what a T1 can provide, it's cheap because it's not guaranteed bandwidth like with a T1; if a lot of people in my area are sucking up bandwidth, I get less than that. If few are, I get most of what's available, hence major speeds. Same goes for nearly all DSL and cable connections.
As for the original issue, I concurr that it's more likely the computer than the connection.
You mean 100 Mbps? m=milli, M=mega.
