In case of questions where the bounty amount expires, I believe the bounty is supposed to be given to the highest voted answer after the bounty was announced, with atleast 2 upvotes.

However, how does it work if no such answer was created?

I am specifically asking w.r.t this question

Unable to install Windows XP : Blue screen error pops up before "Starting Windows Setup"

where the bounty was awarded to harrymc whose answer was 1 hour before mine, both have 2 upvotes and both were posted before the bounty.

However, the bounty was awarded to harrymc. whats the reason behind that?