Are you running any other programs at the same time? I have encountered a problem with some versions of acrobat, where it appears to freeze but it actually hasn't. The problem occurs when acrobat detects an available upgrade and prompts you whether you want to upgrade. If you are looking at something else when this prompt appears, it actually pops up beneath other screens and cannot be accessed, thus freezing until you click on the button you can never access. Try disabling automatic update in acrobat. I think you have to run the reader manually to do it.
