One obscure problem (it happened to me): Some applications install Reader even though it's still there. It is very easy to end up with two versions on the PC at once. I had both 4.0 and 5.0 installed (on purpose by IT group) and, even with the correct options selected in 5.0, it would not open PDFs until 4.0 was fixed. I deleted them all and reinstalled only 5.0. My IT people tell me that may cause a problem with reading some old documents but it hasn't happened yet.