I did three clean installs, both PoP and Warband. The last two were with freshly downloaded PoP.
I tried both fresh installs, as well as installs from files that I had on my mobile hard drive (install backups) which had also previously worked fine.
I even formatted my Windows 8 64bit into clean install Windows 7 32bit, because of the same problem. But for some reason it presisted.
The only thing that could change would by that my second partition isn't of letter D, it's now of letter F, but I can't really see relations with it.
I tried compatibility tweaks, running as an administrator, even disabled display mode. Anybody has any clue?
<EDIT>
No further responses for a few days in a row. Well, here's how I solved it any way.
I formatted my whole computer, installed 2 different Windows systems on the same PC. Tweak(ed)MB on XP, then play on Win 7. So far so good.