Get yourself a 1Gb pen drive ( 70 squid) and load it with Ccleaner, regseeker, and all manner of fixes/freebie programmes (a current version free version of AVG is also handy). It is worth putting the Tiscali dial-up programme on as well so that you can get onto the net if their ISP/connection is borked. It is also worth having a set of boot floppies for ALL the popular OS and a SMALL computer tool kit for tearing into cases. A complete range of freeware trojan/worm removers are necessary.
Make sure that you are red hot on home wireless networking.
Belarc Advisor is a good programme to have so that you can identify all the bits on a computer and a copy of SP2 would be another good one. A list of web addresses of all the support/driver sites is also useful You need to know how to bypass BIOS and system passwords (for those that have forgotten or have bought a 2nd hand computer that has a BIOS lock). Finally read all the forums that you can, the same problems are repeated daily. Get to know these problems and fixes and you are 98% there.
Below is a short checklist that you should do to every computer.............
Remove unnecessary programs from the computer startup
Update virus software and check for viruses.
Check for and remove spyware and/or browser hijackers.
Uninstall unnecessary programs.
Delete "trash" files and temporary files .
Remove unnecessary network settings.
Fine-tune display settings and set display to fit monitor window.
Run diagnostic software and correct disk and/or directory errors.
Defrag the hard drive.
...finally, it is worth sending out a short news email to every customer on a monthly basis....virus news and a few paragraphs of general computer interest, maybe an amusing story (keep it family). They may not read it but it will keep your name in their mind. You can also offer a 6 moth check-up as well.