Try giving us a bit more information other than you're running a 64 bit OS. Is it Windows XP, ME, Vista, 7, Bump on a Log? Also what anti virus are you using? I had this same issue, then I went into my AVG control panel and checked the "enable gaming mode" box clicked apply. Relaunched the launcher and presto, it worked.
Remember the more information you give, the more efficiently we can help you!