People are ignorant and mean about paying people to fix PCs. They think PCs are like fridges - buy use - end of story. They resent paying cash to keep the thing going. The same people pay 30-100 per hour to a garage to keep their cars going. People drop a PC with a problem in your lap and fter you have spent hours fixing it give you a £5 bottle of wine "for your trouble". Charge by the hour. Time what you do and be able to justify it. Tell the person your rates ar £15 per hour, £40 per hour or whatever. Ask them if they want to be phoned if the job starts getting expensive. Don't give your time, money and expertise away for for a few crumbs. Make sure the person knows your charging structure before you start work. They will probably go away and not come back - which is what you want really

The key point is be be very certain in your mind what you want to sell an hour of your life, experience and skill for.
Quoted for the truth! Well said EffBee.
I'll reiterate that it can easily take up to 3 hours to install and update an os. SPW you said it shouldn't, but it can and will if you have a big hdd and a slow broadband connection.
Then if you put say, office 2003 on and then have to update with the latest service pack 3, you're there for quite a spell, whether you can leave it running or not, you're job isn't done yet and you should charge for it.
I wonder how many of the posters here actually do this for a job.
I built a pc for a friend 3 years ago now. Didn't charge him a penny beyond the price of the parts, so i made nothing on it. didn't hear from him in all that time, then 2 weeks ago he rings me up up cos its broken.
To cut a long story short; with all the problems his pc had, i ended up reinstalling XP and Office 3 times. And after replacing the hdd, motherboard, doubling his ram and overclocking the cpu i charged him £60 for it. Which was profit for me as all the replacement parts were my old stock, but then he only had a 1300 Duron and did nothing but look at dating sites, lol.
See what a computer shop would have charged for that!
Don't sell yourself short guys, a monkey may indeed be able to install an os, but some folks can't and will/ought to pay for it.
Mechanics are called 'grease monkeys' but i pay these monkeys to fix my car, because i haven't a scooby doo what to do with it.
I don't charge much because i'm soft, but i flippin well should thats for sure.