sir when i open youtube videos for watch then my pc restart on the sppout and show blackscreen?
Go to control panel, programs and featuers, find FLASH in the list, uninstall it. Go into your browser, uninstall ALL plug ins, or even better uninstall the browser itself.
Now, make sure you remove any "toolbars" or "SEARCHBARS" using same method as you removed flash.
When you are ready, open your internet explorer, download Mizilla Firefox, install it and set to default browser.
Next open Mizilla Firefox, go to google.com and search fro FLASH player, it will take u to the FLASH age and follow instructions to install.
Once everything is installed, restart computer and try youtube again.
^That's the fix for a common Youtube problem, but the OP says his system restarts and goes to a black screen.
OP, please provide your full system specs, including the brand and model (not just wattage) of your PSU. How old is the system? When did this start?