Since my Dell Inspiron 530 desktop c. was upgraded to Win10 Pro, it crashes without any warning, or it freezes, but before that I get many messages like this "Display driver nvlddmkm stopped responding and has successfully recovered". Sometimes though I get the DPC watchdog violation blue screen. It seems that the issue is graphic card (or sound?) related because it happens whenever some relatively stronger graphic use is on work (Skype, Firefox/YouTube, MS Solitaire Collection); nevertheless it occours here and there also during very normal activities. My Nvidia GeForce GT 430's driver should be the last one (355.60 - 2015-08-06).
Strangely, in Control Panel/System the Windows version is Windows 10 Pro, whereas in Control Panel/System/Advanced System Settings/Startup and Recovery the Default OS is Windows 8.1, not even Windows 8.1 Pro which was my preceding OS.
I have no idea how to solve this issue.