The screen is full of green/purple patches of pixels that expand when you scroll down on anything, I have tried anothe monitor on my pc and it's the same result so I think it's something to do with the graphics card.
Uninstall and reinstall the graphigs drivers (get the latest driver if you can).
My PC sometimes fails to boot up, the 'busy' LED light stays on and doesn't flicker. Also the PC will restart itself sometimes, this has happened more recently when I've tried to run Windows Media Player. I have one game installed and sometimes my PC fails to load a saved game whereas other times it can... could all this relate to a hard drive problem?
Download thw HDD manufacturer's diagnostic tool and run them to check the drive.