DIdn't bother reading most of the thread, so some might be a repeat. Antivirus software, but don't forget to have the latest codecs in case they can't go to the internet and update or are just too damn slow.
Partition Magic and such programs are usefull too, as well as those little startup disks that let you view NTFS partitions in Dos.
Whenever I did this (summer job) I always brought along my laptop which had everything loaded on the hard drive and the ability to connect to another computer through a network, direct usb connection, or parallel. It also had an external usb enclosure in case I needed to work on their hard drive from my laptop for some reason.
It is a lot faster to run diagnostic software from anotehr hd then to run it on a computer that is already fvcked up. I also had a spare video card, sound card, usb-ethernet plug, network card, floppy, cd-rw/dvd combo drive, hd, power supply, and ide/sczi cables in the trunk of my car.