Which programs are must haves for a computer repair business?

I have found that the Trinity Rescue Kit (TRK) Trinity Rescue Kit | CPR for your computer to be one of the most helpful tools in my "sysadmin toolbox." TRK is a free live Linux distribution that aims specifically at recovery and repair operations on Windows machines, but is equally usable for Linux recovery issues. The number of useful things you can do with TRK is amazing.

One that keeps track of your customers names, addresses, email and what kind of computers they have, what software and versions are installed, etc. A software and hardware audit program would do well for this.

Compare this to the car dealership that sends out notifications that tells you are about due for your 30,000 mile checkup.

You can use the same type of approach to send out email notices for critical OS updates, software upgrades, etc. to reach out to your customers as a service and to gain more business. If they know that you have their interest at heart, they are going to give you the kind of advertising that money can not buy - word of mouth with a good reputation.