I think something is wrong with my Windows or something... Every time I try to use the patch, manual or in-game, it auto crashes when trying to start up BNUpdate.
Which Operating System are you using? You have updated game before without errors? Sure it is Installer problem? Contact Blizzard Tech Support if it is only Installer problem. I would try to tick box 'Run this *.exe with Win XP settings or something, though this assumes you can access *.exe file, maybe search where Updater *.exe is and modify its run settings.).
You can always download from Blizzard stand alone patch and install or latest patch (Dunno do they allow to download only latest patch alone or you have to get resent stand alone instead.).
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.