Well this is a list of tools I use for fixing computers.....if you want to get serious and do it right...here's what you need...
First all of get a spare computer and maybe a kvm switch...that way you can use the spare computer to scan for viruses and the kvm switch to install windows while still being productive/unproductive
Cat5 Cable crimper....making your own cables is such a plus always convenient invest in a good one, it will be way worth your money.
Cat5 Cable tester....worth it's weight in gold.
Wallet of CD's.....CD's of service packs...Norton Ghost....Norton Antivirus...Misc Programs...and OS's...and spare legal versions of windows to sell
External hard drive enclosure for 2.5" hard drives....great way to back up hard drives
Always have screen cleaner and air in a can
Make a screw kit of Misc computer screws, standoffs, artic silver and etc.
Tubberware containers of IDE cables, floppy cables, and power cables....always a lifesaver..
Good screwdrivers....and/or a gerber multi tool/leatherman
Usb Drive....great for quick backup...yet gotta carry the driver with on 98 machines....
If you can think of anymore please feel free to add