Keep in mind that you're probably not going to compete on price with the likes of Dell and HP. Where you can make headway is with service. If something doesn't work, you can't just say "Hey, that's not our product." and leave it go. You may not sell certain software or be a programmer, but if a customer, or potential customer does ask for help, you're going to need to give them that help. This isn't to say you'll have an immediate answer, but if you're genuinely willing to take a look, you can win them over. (Obviously, this doesn't include those rare turds that want to cause hell, but most of the time, customer service plays a big part.)
If you provide great customer service, not having the lowest price won't be so bad. Sure, you'll loose some potential customers, but not as much as if they hear about your service being bad. Put service first, followed by pricing. People are willing to pay a little extra for something if the service is good.
Depending on the scale, you may also want to look into insurance in case you run into trouble. I would make sure to specify what is and is not covered with repairs. I know the stores do not warranty against data loss. While I don't know the reasons behind it, I would guess that it's to protect the business from that type of thing. It's awfully hard to guarantee data recovery on a bad hard drive when you don't have the tools to do so. There's just a lot of potential liability that I see with guaranteeing data there.