i'm having with my PC which is an HP Pavillion a6700y Desktop PC. In Dec.2012 I upgraded Win. Home Premium 64 to Win. 8 Pro, I did clean install everything went well, i've had no problems until I went to start it up & then it said that the PC had problem & started diagnosing & then a blue screen came on with stop error code: 0xc000021a. I have gone through all the solutions on both Windows & Microsoft & nothing worked, the PC will not even start it goes as far as the advanced options, restore pc, reset your pc, & command prompt which i did all the things like Dr. Watson which I did'nt find, the default debugger, the msgina.dll, & chkdsk & whatever else they suggested. I don't know what else to try,