Adobe have not publicly issued a 64 bit version of Flash to use with 64 bit Browsers. There is an "At Your Own Risk" Beta available for Linux. But not for Windows.
You would think that in the 5+ years since XP64, Vista 64, and now Win 7 64 Adobe would have created a 64 bit version of Flash. But they haven't.
The solution is to use a 32 bit browser for Flash content.