Whats up with my computer?

Start of the year i tried to install SP3 for Windows XP, but for some reason my computer wouldn’t accept it ( its a real Windows pc not a copy ) so i had to revert back to last known working settings. Now when i start up the computer it loads desktop as normal, then after a few seconds the screen blinks ( goes black ) then blinks back to desktop . What is up with that i don’t have any virus nor maleware or such things that can be found anyway . And the settings has not been touched ANY IDEAS. ?