You can, but unless you work as an agent for someone like dell (and I doubt even they make all THAT much really), the only way to really make money is to avoid spending a whole day on each site fixing one machine, and spend all day at home fixing 4, 5 or even 6 machines instead, if you can drum up the buisiness.
Three things keep custom once you have it -
1, Knowing what you're talking about or at the very least appearing to be a computer genius if you don't know what you're talking about.
2, A bit of charisma never goes amiss. Make them like you, make them feel like they can trust you (and don't abuse that trust)
3, A quick, clean, cheap and reliable service that the customer finds to be good quality and/or value for money.
Of course, you have to get it first - the best thing for that its advertising, but more importantly, word of mouth.
This may all sound like a bit of a con job, but remember, it's not. You are not purporting to give the client anything which you're not giving them. It's just business.
The real money in computers however, exists elsewhere. To make it, you have to be either lucky, a computer guru, be better than everybody else, or else go to college for a few years while ignoring most things you learn on pper and trying them out for yourself in practice (nothing like experience) before starting at the bottom/middle of the corporate ladder and working your way to the top.
Or of course you can learn like crazy and become an IT consultant. lol For that you need no experience, just a lot of time to read up, and a few bits of paper.
At this point in time, I should add that everything I've just said about the real money in IT beyond repairman status is all speculation and presumption, just to air my views
lol I've never worked at that level, and nor am I ever likely to (unless I become a consultant. lol)
