I just ran into the same problem about a week ago. I tried to figure out what I may have done to cause this. My computer is 1.5 yrs old, windows 7 pro, i7 quad core with 6gig ram, ATI 4850 graphics card, so handling the program is not a problem. I was editing jpeg photos with SD video. Ran numerous renders prior to this happening and never had a problem.
After careful consideration the only change I made to the computer was to the system restore. I made the system restore capacity larger. I decided to changed the size of the restore back to the original size, ran the render several times & it worked each time. I am not absolutely sure if that was the cause, but it seem awfully coincidental.