If you are using Adobe's CS4 products, there are a lot of ways the licensing can get messed up. If you're licensing goes bad, you'll get an error screen when you start a CS4 product that reads in part:

Licensing for this product has stopped working

You cannot use the product at this time. You must repair the
problem by uninstalling and then reinstalling this product or
contacting your IT administrator or Adobe customer support
for help.

Error: <error code number>

There's a link at the bottom of the message for support from Adobe. But let me save you some trouble, the real link you want is this: http://kb.adobe.com/selfservice/viewContent.do?externalId=kb405970. On this page they explain all the various ways the licensing can get messed up in CS4 and the steps you can take to fix the various problems.