Is it Reader 5.0 or Reader 5.0.5?
I would suggest to update to version 5.0.5 because it fixes Office XP and Windows XP problems.
If you have 5.0.5, no idea what your problem is. But I would suggest to delete and reinstall it and see what happens.
"I think computer viruses should count as life. I think it says something about human nature that the only form of life we have created so far is purely destructive. We've created life in our own image." -- Stephen Hawking