This is a rolling update post for responses to this morning's post.
The Admirable Adam Atkinson has emailed to suggest an answer I hadn't considered: 100.
"I could imagine many programming languages would say 100. You start with the first term, 100. discover that the "next" number, 101, is outside your range of summation, and stop there. I don't know that I'm saying this would be my answer but I could imagine some people supporting it."
I agree, it would be perfectly possible for a computer language to interpret this as something like:
total = 0
count = 100
total += count
count += 1
until count >= 0
... although I don't know any languages off the top of my head that would do that. Adam notes that Perl considers the sum to be 0.