I seem to recall that at some point last year Nvidia released a driver that was causing a pink screen issue on 5xx series cards. The pink screen would occur when starting games or watching Youtube videos. I think it was version 296.10 that caused the problem and it wasn't fixed until 301.42. Since you mentioned that you've reinstalled Windows, I assume you've installed the latest drivers too?
Some people also seem to mention that GPU monitoring/tweaking software could sometimes be the culprit. Some cases of the pink screen issue seem to be solved by uninstalling software like Lucid Virtu. I kind of doubt that's the problem here though.
Pink screens can also be caused by bad monitor cables and connections but that of course seems not to be the case here due to the timing of the issue.
When you mention the video issues, are you speaking of Youtube video? The videos usually use Flash and as a sort of temporary band-aid until you solve this issue you can disable hardware acceleration for Flash. Right-click on a Youtube video (or any other Flash window) and choose "Settings". From there click the left-most tab on the bottom and then un-check "Use hardware acceleration". That should at least allow you to use Youtube until you get to the bottom of the issue.
Wish I could offer more help. Good luck!
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