Show a screenshot that includes your Timeline panel and Preview panel.
It might be unrelated to your problem, as I'm running AE on a mac, but it could also be.
This afternoon, I've experienced same error (first time ever), also tried to purge, to reset work area, and went to quit and relaunch AE. Problem was solved immediatly, and didn't occur a second time.
I'm using AE 12.1 on a OSX 10.8.5 / 32BG RAM system.
You need to have much more RAM reserved for other applications and more CPUs reserved for other applications. Your current settings are crippling your performance.
After you have adjusted your settings according to the recommendations on those pages, restart your computer and try again.
I went ahead and clean installed my computer and did what u said. Havent had any problems since