Moving this discussion to the Adobe Digital Editions forum.
Which operating system are you using on your PC? Do you receive any specific error messages when you try to install?
If you are on Vista, see the announcement at the top of the forum,
which points to http://helpx.adobe.com/digital-editions/kb/installing-digital-editions -20-windows.html
Try an older version (1.7.2) of ADE, especially if you are on XP.
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.