I do a good bit of tech support for people these days for a little bit of extra income. So a few months ago I made up a kinda of tech-emergency-kit. It's sort of like a first-aid kit, but for computers and such. It's still changing depending on what I need, but so far it really hasn't needed much more for any of the situations that I've come across. Some of the items here you can do without, pretty much all of it is cheap and I'll provide some links for as many as I can.
This is really up to your discretion. I use a tackle box and a regular toolbox to hold everything in and twist-ties to keep things tidy.
List of items and their use:
* 4GB-8GB USB Drives x 2 = Any brand is fine, the cheaper the better. I always have two different brands just in case a computer won't read it or one messes up
* 120GB or larger USB Drive = This is essential if someone computer is so bad off it needs to be wiped clean. Use this to make a backup of everything that needs to be
* VGA to RCA converter = I personally use this instead of carrying around a seperate monitor. It's main purpose is for people that don't have an HDTV with a VGA hookup.
* USB/PS2 Keyboard = For backwards compatibility
* USB/PS2 Mouse = For backwards compatibility
* USB Hub = For using the Keyboard/Mouse
* Wireless Router = Check Woot if you decide to carry one, I got my spare one on there for just under $20
* Outlet Strip = Handy if there is a lack of plugs
* Lots of cables = Have at least two of each of the following:
7' Ethernet, RCA 3.5mm splitter, 12' RCA 3.5mm, 12-25' RCA 2 cable, RCA splitter x 4, KVM cable, Phone cable, Power cables 1 2 3, HDMI to DVI, HDMI, 6' Component, 6' Composite, 6' Svideo, 6' VGA, Universal USB cable, Univesal Laptop power cable
* Electrical tape
* Boot CD's of Ubuntu, Windows XP, Windows Vista, Windows 7, Ophcrack
* A couple of spare CDs and DVDs
You'll need a few handtools as well (screwdrivers mainly), but none that you shouldn't be able to improvise with. Most everything on here I got for free from different places and people. Just take your time building it up, starting with the Boot CD's and USB drives, everything else is extra. With all this stuff you should be able to fix just about any computer or home theater.