Same problem with audio; same fix disabling modem works.
I have the same problem with a dell inspiron; I reinstalled Vista and the sound is not installed (and frustratingly, I can't turn off the system sounds! - the speaker is functioning well enough for these!)
Disabling the modem and downloading new drivers seems to have repaired the problem. But, hey, Dell, does this mean your customers have a choice of a functioning modem or functioning audio, not both? I hope a fix is in the works!!