In all honesty this is almost an impossible question to answer - I have been doing this for over a decade - and I'll stumble on a new cool tool by accident when googling for some other things.
another thing I have been playing with is portableapps - there are a lot of useful tools that you can use that require NO footprint on the machine you are fixing - they have an antivirus scanner - which is sometime a godsend - (just make sure to keep it up to date with the virus defs.)
I ran across this site too - with a few rescue CD ISO's for virus scanning - not sure how well they work - as how would the definitions get updated in an ISO - but definitely something to look at http://www.techmixer.com/free-bootable-ant...-download-list/
Good Luck with your business!
***EDIT*** find yourself a good undelete program - this is something that will prove invaluable.
I used a program the other day to recover my mileage log called RECUVA and while it didn't "undelete" perfect Excel files, it looks like it may have recovered just enough fragments for me to put my spreadsheet back together
Edited by zarraza, 22 June 2009 - 02:39 PM.
Mod Edit - Removed Link To Password Breaker Per Rules