comcast does dsl? aren't they a cable co.?
first off, it sounds like you went with a USB modem. Ditch it and get an ethernet-based modem. No drivers, no crap to install.
as for removing whatever comcast installed, you might have to boot into single user mode and do a bit of rm from the terminal OR hook your powerbook up to another mac via firewire and boot the pb into fw target disk mode, from which point you'd be able to ferret out and remove the comcast-installed files.
since when does comcast give anyone a cd-rom with a mac os x installer? I've never seen that.