The System Rescue Linux Boot CD (I have version 1.04 and it works fine).

This is needed to break the Administrator password on NT, 2000, XP and Vista. Often there isn't one, but if there is the client never knows what it is.

This disk is also essential for going into the filesystem and removing files Windows can't remove (which now includes some Adobe Reader files), renaming the Windows directory to get it out of the way and other similar uses.

I also find ZTree very useful at times (it's not free). ZTree development began when XTree was bought and abandoned, first on OS/2 and then on Windows.