Day before yesterday I had downloaded Windows 10 iso using "media creation tool" mine is a 64bit OS & yesterday upgraded my windows 8.1 to Windows 10. After completion of installation Windows 10 got activated & I was very happy with Windows 10 & decided to delete the "Windows.old" folder by using "Disk cleanup" tool everything went fine & started installing my much needed softwares & drivers. VGA drivers include "Intel graphics driver & NVidia GeFroce 820M graphics driver" & installed "Wamp Server" along with that I had also installed this "vcredist_x64" because it needed one dll file I forgot its name & after that I had downloaded & installed latest version of free BitDefender antivirus, after its installation screen went blank & came back since then Start Menu & other things such as notifications, sound control are not opening. I don't know what all happened in background. Start menu is not working :-( I'm beginning to think that Microsoft has released an half baked OS to general public.