I got my PC back from repair (was using my laptop instead of the PC for half a year now until I found time to get my PC repaured) and installed Pandra's Jar. I was really dazzled when I found it not working properly, although the settings where similar to those on my laptop. It DID rip the mp3s, but not in the specified directory, but in a folder named 'unclassifieds', not being named with Artist - Title but with their MD5 checksum.
I had installed PJ on another partition (E:/pandorarip), but it was a fresh installation, not just copied from the notebook. Just copied the default.properties over and adapted the path to E:/pandorarip.
I than tried something I'd never have expected to do the trick:
I installed to C:/pandora (like on my notebook, where PJ was working all the time) and adapted the path in the default.properties.
And now it works like a charm!
The only possible reason for this I can think is, that C:/pandora is is hardcoded somewhere, and therefore running PJ in another directory won't work. Maybe CF will have a look onto that issue or has another clue why this would happen...
So for all those having troubles:
Try using C:/pandora and setting mp3.archive.path=C:/pandora/
and please report if that worked out for you as well!