Desktop

Ultimate Boot CD is a great resource for troubleshooting tools and other programs. I always have a copy on hand. It also is fairly easy to customize. http://www.ultimatebootcd.com

http://www.answersthatwork.com/ gives a list of drivers for all makes and other stuff pretty neat site.

cm is right but I went up a step and got the ubcd it is a collection of programs, some free ware and some shareware. This is a live cd it runs off the cd not the hard drive so you can run antiviral, spyware removal just about any thing you want and it will run on Win/xx, 2000, XP, 2000/2003 server. Also look in down loads there are a few down loads there for the same thing.
PM me and I will give you a list of things that I use, it is too much to post here.

As well as TUBCD it's also a good idea to have a couple of Live Linux CD's to boot from as I've seen a couple of faulty M'Boards that will not load Windows but will run perfectly on Knoppix but not any other Live Linux. Knoppix also gives you the ability to remove user passwords from Windows when the user has forgotten their password.

As for hardware tools a Power Supply Tester and a Digital Multi Meter are an absolute must. If you are doing any networking jobs a cable tester is also very helpful. I once got caught out when someone replaced a 50 CM Patch Lead and the entire system went down without the cable tester I would never have picked up that they had accidentally substituted a Crossover cable for a straight through CAT 5 Lead. I just hate it when people are too tight to buy the correct Cable to make these leads. Or even worse some of the new makers are making both types of cable in the same colour and once the bits of sticky paper come off it's impossible to tell them apart.

Col

I dont think I could live without PsTools from sysinternals
http://www.sysinternals.com/Utilities/PsTools.html
I use psexec and psservice countless times a day