By default, Knoppix mounts all drives in read-only mode. The Knoppix philosophy is to make it inherently difficult to accidentally mess anything up.
Also, Knoppix 3.1 is a rather old version of Knoppix, which I think doesn't include the ability write to NTFS partitions (Windows XP uses NTFS). The USB drive should be formatted in FAT, which Knoppix CAN write to--so you should be able to take files from your WinXP partition and copy them to the USB drive. However, you'll need to right click on the USB drive icon and change it from "read-only" mode to "read-write" mode.