I am trying to find out - if I have a population of 17,920 with an average score of 1161, what is the population whose score is over 1,000?

It's imposible to say. You need to know the standard deviation of the scores in order to determine this. If you know the standard deviation s, then the "Z-score" you're interested in is:

z = (1161-1000)/s

Then you can look up the percnetage of a normally distributed population whose z-score is within z of the mean. That gives you the percentage with scores from 1000 to 1161. In addition you know that 50% of the population scored higher than the mean of 1161, so you add 50% to this to ge the total percentage that got 1000 or better.