Try to boot in Safe Mode ( pressing F8 while booting) and form there try to System Restore to the Point when you know it was good!
It's possible that you update your video card driver, but something went wrong, so restoring your system before that can fix problem..
But , it can be problem with your vide card too! You didn't specify your hardware components, so what's your video card?
You said that all cooler are working.. Did you unplug your video card form PCI Express slot and clean dust? Did you remove cooler and clean dust too? And can you know what temperature is your GPU? You can use AIDA64 , or SPECCY application for that. cause maybe overheating is causing those artifacts, so you'll need to clean all dust and put new thermal glue..
If that even work either, it's possible that your video card went 'dark' so you'll need to do extreme measures! First ( and I suggest this!!) to use electronic velder with hot-air option! Then you pass all electronic contacts on video card with that ( couple of minutes) hot-air blower and than lett it cool of for a few minutes, then try to put it back in computer!
Second method is radical method when you put your graphic card into to oven! ( Go to YouTube for more info) and than pray for luck
Or you can borrow freinds graphic card and put it in your computer so that you can be sure that every other hardware works properly ( PCI EX slot, Motherboard..), so then you can go and try fix your graphic card with all these method..
Hope I help with some of this..