it's a nice sentiment, but it's unsustainable and it's setting bad expectations with the client. You should not be guaranteeing bug-free software, because that is impossible. The client should understand that the software will have bugs, and that your ability to continue fixing those bugs is contingent on you having a continuing source of revenue. I'm sure they'd rather pay than have you close up shop and have nobody to fix the bugs.

Also, i've noticed in my own work that people are always eager to blame problems on whatever component is the cheapest/easiest to solve. If you charge $50/hr for application support and the guy they contract to do virus scans and run disk defragger costs $40/hr, their first call will always be to him, not you, no matter where the actual problem lies. Somebody at their office has figured out that whenever a user has a problem, it's cheaper to pawn the problem off to you and let you do the triage than it is to triage it themselves before calling you. Even if you don't start charging for bug fixes, you need to start charging for wasted time.