I want to have enough stuff on hand in my van that I don’t need to drive down to Frys when a customer needs a part. I will be mostly doing home and small business stuff.

I figure I will get the we got this computer and need to get on the internet, get wireless, etcetera.
I saw a million years ago in a Maximum PC what they felt a tech should have with them. But I can’t find it.
I don’t have a lot of money, but want to be prepared. Any ideas of software stuff I might need? Virus/Spyware softeware. Diagnostic stuff? I figure a USB/Firewire external drive for backups and transfer. USB cables, firewire cables, ethernet cables, cable tester, USB mouse and keyboard, PS2 mouse and keyboard. Canned air, blah, blah. Any other ideas?
Do you guys have them sign a waiver before working on their computer, in case there is data loss, or the computer gets fried?
I worked in a box store before this so I always had access to stuff.