For comparison, when I worked as an on-site tech doing service/repairs/builds for a small mom and pop shop back in the late 90s, we charged $75/hr for individuals and $85/hr for businesses. When I did jobs on the side, I charged $50/hr and my customers thought they were getting a bargain.
Keyword here being "the late '90s". Them days have come and gone.
@op: For routine tasks, I would suggest developing a rate sheet that allows you to charge by the job type rather than by the hour. If you set your rates right, some jobs of any given type will take more time and some will take less depending on what condition the system is in, and in the end you'll average out where you want to be. Customers purchasing services don't like vague costs and will try to pin you to a firm estimate, and may not give a good recommendation if you run much over. But if you quote a flat-fee up front, the worst you'll have to deal with is an occasional slippery deal-maker who wants to bargain you down. (Don't even deal with that guy, it will be an endless headache.)
A fee structure frequently used by service shops is to charge the customer a small but meaningful fee (say, $15) for an analysis and estimate, then roll that fee into the total cost of service if the customer agrees to purchase additional services. Since most people don't understand the doctrine of sunk costs, once they've committed the up-front fee they're a lot more likely to purchase the follow-up services provided you have otherwise make a good impression. If you analyze it for free, discover a root-kit infested XP system with only 512MB of RAM, and start quoting prices of $130+ for parts and labor, they're more likely to shop around for other estimates or just say "Meh, I'll buy a new computer."