How do I turn around the poor performance of some of the employees that I have in a constructive way?

The unfortunate thing with small businesses is that they often do not have formal employee reviews. If you did you should make sure on the form that there is a well documented areas for concern with a game plan on what the employee is expected to accomplish.

I agree with Triton with the legal issues we are all faced with you need to make sure that you have a well documented performance reviews.

Regarding how to improve performance, go out of your way to make sure you compliment them when they are exhibiting the right behavior

Confront the employee. There may be a good reason for the individual's poor performance. Then again, there might not be. But it's your responsibility to find out what's going on. Begin by discussing the person's work with his supervisor, peers and the users he supports so that you get specific examples of shortcomings. Once you have that information, sit down face-to-face for an honest and direct discussion about your expectations, where the failings are, and what impact that performance is having on the company.

Documentation is key and getting the employee to acknowledge the problem and getting them to take responsiblity for correcting it.