Unpacking natives fails with access-denied.

I use Windows, and every time I try to use the new Minecraft launcher, I get this message:
Launcher started...
Getting syncinfo for selected version
Queueing downloads
Launching game
Couldn't unpack natives!
java.io.FileNotFoundException: C:\Users\BenJ\AppData\Roaming\.minecraft\versions\13w16a\13w16a-natives\OpenAL32.dll (Access is denied)