I had the same exact issue. To get it to work, I had spam the F2 button on restart to enter the system BIOS menu before windows loads. From there I scrolled down to performance settings, and low and behold, Overclocking was on by default in system BIOS. I turned OC off in that menu, restarted my computer, ran the BIOS flash again, and to my surprise it worked!!!! After I confirmed it upgraded to Alienware BIOS 1.6.0 from 1.5.1, I then spammed the F2 to enter system BIOS again, turned back on Overclocking, as it was set previously, saved, then restarted my system. All has been good since then and hope this bit of info helps you or someone else who may have the same issue.  
So I tried to flash the bios, and it shows CPU is overclocking, so I disabled the overclocking in AWCC and also in BIOS menu, but it still shows the same message, not allowing me to flash the bios, the "ok" button is just greyed out, I'm out of ideas, what should I do?[IMAGE]
Aurora R13, OC disabled, still won't update BIOS