Best Buy charge $200 for Virus/Trojan/Malware scrubbing with another $100 added on if an RRR is needed, with no backing up of stuff. Backups also cost $75+ depending on time required etc...
The guy down the street(*) charges $75/hr and YOU bring it to him. He is swamped. He always gives the people a Linux LiveCD upon completion. Tells them if they have to have Internet or get a file and something happens again, pop that CD and boot from it. First call on that CD, up to 30 minutes is free.
Always leave a Linux LiveCD with the customer. I suggest you choose a Distro and get to know it.
Also the usual gamut of programs, SpyBot,AVG...etc is something you will need on a CD.
Edit:
* == Guy has a SMALL shop on the corner on a busy, but not developed intersection. Cheap rent and lotsa money coming his way. Its a good thing.