I think you just hit on the cusp of doing repair work, the "it never did that before" factor. I do work on systems for some people because there is really nowhere else around here to get it done. First thing I get is a good explanation of what is wrong, and importantly, what they expect me to do, what they think I can do. Usually there is a lot more wrong with the system then they think and if I just fix what they want then they still get back a pretty unstable system. Then I clone the drive before I do any work and keep that clone copy for awhile.
I always have them turn it on and do an inspection of the inside with them there before I accept it for work. Some people bring me say a P100 with 8MB of ram and "want Windows XP" on it. I have seen burnt MB's from no surge protection "wont turn on" I then will tell them what my estimation of repair is and if I think it is worth it to them, often I will just offer to retrieve and back up data to a cd for them and point them to Dell.
But I usually charge only for parts, and make it clear upfront that it may take 30days or more for me to get it done. Its kinda like a favor and at times I just tell people I'm too busy, and I never take more then 1 computer at a time and usually for people I know and like, and I need to have some time on my hands.
The thing is that no matter what I do, almost without exception, once I work on someones system the assume that they own me for tech support. So I am very selective and do not advertise.
I used to try and educate the user, but beyond really pounding home the need for virus, surge,defrag spyware and share/password protection: I leave them to their madness.
(I put the shortcuts on the desk for adaware, defrag, etc. and rename them to "update and run every 2 weeks" or "run weekly")
Good to be back and ramble again