I dunno, i think helping friends out with PC problems & upgrades can be somewhat rewarding.
1) Makes them happy at the end
2) Their PC will actually be upgradable, no onboard graphics, and the PSU won't die when you sneeze at it.
3) Generally from my experience, friends always offer to pay or give you something in return, wether that be beer, dinner or some hard cash.
Of course theres the support side of things, but generally i try to setup fail safes for those not quite capable of looking after a PC, Anti-virus protection, Spyware protection. I advise people to properly defend their PC, wether or not they decide to invest in some software is down to them, but within saying that i like to mention that in the event of spyware or viruses they're on their own if they refuse to pay out for the software that will potential save vital information.
I always explain to people if they want to clean a PC, i'll do a one off fix but after that its down to them to learn to protect their PC effectively or next time they'll be paying near on prices a computer store would charge to fix the PC. This in turn usually means they just let the PC get cramped with crap or that they spend a little time learning about the PC and investing in some good software, again i'm always recommending the ones i find least system intensive, yet still effective and not overly expensive.
Friends do expect more for less, but if you explain what a PC store charges in comparison they will either give you something for it or invest in avoiding PC issues again.
I've fixed quite a few over the last 2 weeks and built a handful to, out of those, i've made some cash, 1gb pc3200 DDR, a case with a sidewindow, a FSP Power Supply, a Sempron 2400+ (its not the best but its free).
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On-topic with the business thing.
I've also considered investing money in doing the same as the thread starter, but am still yet convinced it would be easy to make a good profit without causing too much stress to myself.....
Upgrades for friends and family is one thing, offering a service (with warrantee) is a totally different ball game.
If you want to start a business, like already mentioned, you need to know what area of the market you are aiming for, and then research some time in seeing if you can actually sell to this area and still make a good profit.
On the mentioning of the big retailers getting parts cheaper because of buying in bulk. I don't think you'd be able to get lower supply prices they get, but you wouldnt be far off, you just need to cut out all the middle people and find the right place to buy from. Its easily possible, i know this because my dad spent a few weeks last year researching how cheap we could get parts when he himself was considering a PC business (poor guy couldnt get a loan to start with though), but i do know he found some good sources for supplies, and happens to know an individual at Microsoft that may be able to help. That aside though, as a business grows you'll have more money to invest in more bulk deliveries.
What type of systems would you build though?..... are you wanting to sell set builds, or will you be making custom builds, or both.
Personally for me, the problem i have... is that i refuse to buy in and use parts i would not put in one of my own systems, this of course is good for the consumer, but they of course will not appreciate the quality of the hardware, and not realise the differences between the Nice shiny Tagan or Seasonic PSU and the cheap crap ones in say a Tiny PC. This makes me ask myself if i'd look at cheaper parts, and i have to tell myself NO! i would not. So is this a realistic business for me to invest in, yes if i can invest a lot of money into high quality built systems and find a good customer base.
The way i see it though, everybody these days has a PC, however due to all those dodgy companies like Time, Tiny *used these 2 as examples* they have really shoddy systems that get clogged up with rubbish, spyware and viruses and lack a lot of memory usually. a lot of these people will either choose to have the system cleaned by a professional, or most people just choose to buy another cheap system.
Theres people everywhere with these PC's, and a lot of people have children that don't use the PC because it won't play any games, but sometimes these families will fork out on the latest great console then buy a cheap PC.
Realisticly though, the amount spent is equal to buying a good spec PC, i mean if you asked most people what they think of PC's compared to a console, generally most people will think consoles are much better for games, but i don't blame them when they have a Compaq, or *INSERT NAME HERE* sitting in the corner of the room that cries when you ask it to open a webpage.
I've spoke to various people, customers at my previous job (not pc related) and other people i've met, such as the heating installer the other week, and generally families want gaming PC's but fail to see that a PCworld with 64MB Onboard Graphics is bad

. People that i've spoken to think £400 - £500 is a reasonable price for a PC. So can you make a family gaming PC for that price and make a profit, simple answer is NO!.... unless you cut corners and use cheaper parts.
Its the overall education of the home user on PC's that makes it hard. They see a Pentium 4 on the TV with 512mb at PCWorld for £425 and its already sold. So i'd have to ask myself how am i going to educate my customer base without BORING & CONFUSING the hell out of them.
Its a big subject, and with every corner comes more problems and questions you'll need to answer and be ready for.