Ihad a similiar problem with my dell 4550, after I set the bios to boot from cd knoppix ran fine. But after I removed the cd and rebooted to windows my computer would not boot. Found that there was an error in the memory module. I had to reseat the memory and then windows booted fine. Just be wary when using knoppix on a dell.

And on a different note I tried to run knoppix on an older compaq and found that the monitor is not compatible with knoppix, a compaq specific problem I am told.