Let_me_read I would recommend that you temporarily disable any security software you are utilizing to help determine the conflicting software. The sample book library is likely utilizing a different content server than the any titles you have purchased. The error indicates that communication is not being allowed or interrupted no matter which Content server address is attempted.
You may also want to verify that you are not utilizing a proxy server. This seems to be another security safeguard utilized which can affect network connections from installed applications. You stated you are using Windows 7 and 8 so this can be found in the Internet Options Control Panel. It is under the LAN settings options.
I got the 4.0.1 version working on my Windows 7 machine. I uninstalled again, and then scoured out every Digital Editions related file and directory I could locate in the user and app settings/data subdirectories. Rebooted. Re-installed 4.0.1. Got a new error when trying to authorize the computer which led me to delete a registry key as indicated on this page: http://helpx.adobe.com/digital-editions/kb/error-e_act_not_ready.html
Started ADE up, authorized the computer and was then able to add books. No network settings were changed. No proxy servers were running. 4.0.1 ADE on my Windows 7 now adds books from .acsm files.
Still getting the same E_IO_CANNOT_OPEN error on my Windows 8 machine after trying the same process. I suspect whatever files were lurking after uninstall that I managed to purge in Windows 7, and were causing whatever conflict leading to the error, are still out there somewhere in the file system.