Downloading Digital Editions

How come I can download Digital Editions 1.7.2 but not 2.0.1 or 3.0?  Now 2.0.1 and 3.0 will download to my computer but my computer will not recognise them and If trying to acces them the computer locks up.  1.7.2 runs and downloads books from Public Library, but the Library really wants 2.0 to be used.  I sure would appreciate any help.  I have been using 2.0 for well over a year but had other computer problems and it got screwed up in the process.  Computer is great but having the problems above.  Thanks you for any help.