I have recently purchased the DWA-556 and had random blue-screens and restarts on Win 7 Pro 64-bit.
After searching a bit I have found that if you update the driver for DWA-556 within Windows to Atheros drivers, the product works fine !
I have downloaded these drivers from:
http://www.station-drivers.com/page/atheros.htm
The AR5*** / AR9xxx is the one that I have used (I actually followed someones post about this and can't find that URL).
Give it a try, and I hope that you find this helpful.