USB ADSL modems are well known to cause problems with Macs, especially sleep related problems.
I use to have an Alcatel jobbie that used to give me kernel panics (the multi-lingual grey screen you're seeing) on a regular basis.
I've had a recent Powerbook for almost a year now and have never had a single crash (well, apart from Logic!)
Best bet is to dump your USB modem and get a wireless ethernet one, available for about £50 now. Once you try wireless you'll never go back.