The app on my computer is authorised, however, when opening a file and error message appears saying I have to check the activation, which then appears to be ok.

Macintosh:

1. Open Adobe Digital Wditions.
2. Press Command+Shift+D to deauthorize Adobe Digital Editions.
3. Select Erase Authorization.
4. Click OK and quit Adobe Digital Editions.
5. Navigate to /Users//Library/Application Support/Adobe/Digital Editions and drag the activation.dat file to the trash.
   If you are using 10.7, see Access hidden library files | Mac OS 10.7 and later.
6. Open Adobe Digital Editions and reauthorize.

Windows:

1. Close all applications.
2. Choose Start > Run.
3. In the Open text box, type regedit and then press Enter. The Registry Editor opens.
4. In the left pane of the Registry Editor, locate the following registry key: HKEY_CURRENT_USER\Software\Adobe\Adept
5. Select the Adept key.
6. Choose File > Export.
7. In the Export Registry File dialog box, select Selected Branch under Export Range. Enter a name and location for the backup registry key, and then click Save.
8. Right-click the Adept key and choose Delete.
9. In the Confirm Key Delete dialog, click OK.
10. Close the Registry Editor.
11. Open Adobe Digital Editions and reauthorize