Think about your goals. There's no reason to go into an intense class if all you're looking for is a jump-start. If your goal is to get a promotion and the new position uses Excel, take a course in Excel instead of enrolling in an administration program.
Look for reputation. Computer training is offered in a lot of places--look for a school or group that can give student references and has at least five years in business.
Ask about the instructors. Good computer training should be offered by professionals with a great technology background.
Ask about the technology. After all, what are computers? The software, equipment, and supplies that you learn with need to be up-to-date and industry standard or you're not going to be able to use your new knowledge.
Look for hands-on experience. The best way to learn computers is to use them. It's hard to learn what you need to know from a book if you can't practice on a computer as well.
Aim for take-home material. A certificate is nice but is only as good as how employers view it. What's better is if you have manuals, training discs, and a student portfolio or examples of work that you get to leave with.
Find the time. When are the courses offered? The best training will do you no good if you can't make the time to be there.