I've gone through the tickets for calibre, the closest match is this one
(look at the jpg), though it is on windows 7. According to Kovid, it is "is most likely caused by a conflicting install of Qt or some program that uses Qt".
Would that make any sense? Do you use any Qt programs at all? If that doesn't mean anything to you, would removing the old calibre in its entirety be an option before installing this one?