difficult to read ebook on different pc

Hi, I haveinstalled and activatedADE 2.0 on a PCwithWindows 7 and on a pcwithwindows XP,to readan ebook purchased from Koboonboth PCs. However, whileI can read well,  in the first pc, the characterwith whichthe text is written,on thesecond PC I can not read(althoughI canopenthe ebook), because it isas if thecharacterwaspoorly written, poorly defined(with a few pixels). Can anyone tell mewhat it depends on? Thanks