Lots of good advice.
I was in a similar position to Mardell a few years ago, I have worked in IT for years and was working for a large company designing/installing medium size corporate networks and school/college networks and such. I was forever getting asked to "come and look at my pc" which I did and I enjoyed doing. I often thought of setting up on my own - then I got made redundant and got the push that I needed.
Its not easy. Home pc repairs dont bring in a lot of money, at say £25/hr over 8 hours (ok, we work more than 8) thats £200/day but of course its doubtful that a single job would last all day, so you have a few 1-2 hour jobs, then theres time travelling to them, petrol etc - doesnt leave a lot after tax.
I do home pc's, but most of my time is with businesses and one day I hope to stop doing home pc's totally. BUT, and heres a bit of thought, one of my best corporate customers actually came from repairing a home pc. The persons pc I fixed was really chuffed as they had already had several people around trying to sort it. It turned out their son is the MD of a fairly well off company in my local area. Soon after I got a call and that was that, another regular(ish) decent income on the books.
One thing that has been mentioned is stick to what you can do, i.e dont be a jack of all trades/master of none, if you cant do it then walk away. I often get asked about we design/hosting/email etc, I'll help with hosting, usually by reccomending Carl (CS) I'll also help with mail and simple changes to their site, but anything major to do with web design I wont do, simply because its not what I do, web designers do that. Likewise if you dont know how active directory/DNS works then dont go near a domain controller till you do.
Leaflets are a funny thing, you give a few thousand out and generally get a couple of calls within the next couple of days, then nothing. But, later on you may get a call from someone who answers the door waving one of your 1st leaflets at you, all screwed up and yellowing from age. A few people do keep them.
I found my yellow pages ad works well, although quite a few people on here wouldnt touch it with a barge pole. My ad costs £400/year and in the 3 years I've run it I've always had my £400 back from it.
Just a few thoughts there. Back to the original question, home pc calls here have got less and less since vista was released - I HATE it, and so does just about every technical person I know, but, jo bloggs doesnt seem to be able to break it too easily.
Theres the memory upgrades that have been mentioned, crucials ads about the pc running slow so throw some memory in seem to work - often though as I'm sure you'll agree, its sometime like taking your oil pressure light out to stop it coming on - i.e. not getting to the root cause. I regulary get asked to do an upgrade, I'll look at the pc and more often than not all it needs is a bit of a clean up. I could of course fit some ram and charge, but I'll usually explain why it didnt need any and how I've saved them money - another happy customer who will pass my details on - as long as I leave some business cards, and of course one of my mouse mats and coasters
