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That isn't a virus. Your problem is caused by faulty hardware. Likely a video cable or screen.
 
Oops, I meant it hasn't occured since.*

Use the hardware test that comes on the install disks that came with your mac. That could give an indication as to whether something is faulty with the hardware or not. Also, if you have it, a test with tech tool deluxe might be worth it, but don't worry if you don't have it. If it happens again, or results of the test show something, then I would take it in to apple. Otherwise, I would just dismiss it as a one off glitch/blip.
 
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