I had a similar problem with spss 15.0 (with patch). I had windows vista and about every other week I got an "unable to acquire license" error. At that point, I had to uninstall and reinstall, which would fix the problem for a week or 2. Because of this and other software issues (many, many) i switched back to xp, where it works perfectly.
If I had to take a wild guess, I think after x amount of time, something got out of sink with vista (maybe a time signature or something) and no longer recognized the license as valid.