Knoppix defaults to opening partitions as read only, but it can easily change to mounting as read/write if you are talking about a FAT partition. If the partition is NTFS than you stay with read/write (5.0.1 claims "limited" ability to write to NTFS, whatever that means, but I'll wait for others to risk their disk on this bleeding edge technology and report back before I try or advocate that). You didn't say which partition type you have.

If the disk has one or more FAT partitions and you just want to erase the old data then see answer #6.

I suggest however that you delete the partitions on the disk entirely and then create a new partition and format it with whatever format type you select based on the OS or OSs that will be using it. If using windows I would use the windows tools to format it, particularly if use XP and want a NTFS partition.

It would be wise to rewrite the MBR as well, on the chance that there is a MBR virus hiding there. This would not be a problem if the disk stays the secondary drive, but if it ever becomes the primary drive in a system down the road then a lingering MBR virus could come back to haunt you.