conflict between 4100 dsl modem and 3com 3C920 adapter?

I just bought a refurb Dell Latitude C640 w/Windows 2000 Pro and I'm having a problem with dropped internet connections. I have a Speedstream 4100 dsl modem from SBC/Yahoo connected directly to my laptop. After several calls and trouble-shooting sessions with SBC Tech Support over the last 48 hours the last tech I spoke with said that it appears my internal network adapter (3com 3C920) is overriding the dsl modem. Assuming the tech is correct and such a thing is possible, how do I correct the problem. Any suggestions?