Don't do house calls
My response to these annoyances is to tell the individual in question to "bring it over and I'll have a look". You are then at liberty to leave any computer that does actually turn up (the prospect of having to do something, themselves, seems to deter most people) in a dark corner for several weeks. If the owner does summon up the courage to enquire, just mumble something about "cross wiring the Southend bus" or somesuch and tell them you'll be in touch.
The problem with going round to their house to fix a problem is
(a) it puts you under pressure to do something immediately
(b) you're away from your tools, favourite debugger, reference sources and possibly even a viable internet connection
(c) unless you do manage to randomly fix it, it's difficult to get away when you've had enough
(d) it takes no effort on the part of the ask-er - always a bad position to be in.
The other advantage of leaving someone-else's kit to fester while you're "fixing" it, is that you could build up a stockpile. So when yet another freeloader cheefully asks for hundreds of pounds of your time (at professional rates) for nowt (or worse, in exchange for a bottle of undrinkable wine, that they were probably given on the cheap), just point to the pile and remind them that you've already got Fred from No. 6's to do first and then you promised the kids from across the street that you'd sort out their video drivers ...
Alternatively, you could just hand them your lawnmower and suggest that while you're fixing their PC, they could make themselves useful with the jobs you were about to do, before they interrupted you.