Just to mention it, there was some issue reported with Windows 8.0/2012. The symptom was that customization would get stuck when applied to a just cloned VM which not powered on before customization. I personally have never seen it repro with many templates we've tried. Anyway, the fix was backported and will be available in 5.1.3 (and 5.5.0 GA as well).
/Andrii