Also having the same problem. (but with a PRS505)
Had bought and downloaded a couple of books through ADE. Wanted to put them into the folders i have on my SD card. So as per usual shut down the library, unconnected the ereader.
Reconnected, library pops up, wants to download an update. Click okay, now the download froze before finishing, which seemed to remove the library from my computer. No probs went to the website to download. Seem to install fine but the library just will not open.
Also tried uninstall, reinstall, system restore, the DEP thing, using the original version from the CD (which opened but wouldn't recognise that the ereader was connected to the computer). It just will not launch. It sits in task manager but no programme opens up.
As far as I can tell it's nothing to do with the ereader itself as the library refuses to open even when it's not connected.
Stayed up to 12 last night trying to fix it to no avail