Anything bad in the title or comments about Microsoft or IE results in automatic upvotes regardless of content.
But you can't have it both ways: if there's only a tiny minority of non-Windows people, they won't have the juice to generally influence Microsoft-related article's ratings. It you want to have it both ways (Windows dominates to the exclusion of everything else AND those pesky Linux/Mac/whatever users are everywhere, upvoting critical articles), you've got a world view with a very basic contradiction at its core.
So: are there more Mac/Linux/Minix/LoseThos users, or do even the Windows users generally dislike Microsoft?
First, I'm assuming that Windows-users are represented proportionally with the general population in the Hacker News population.
Second, I'm assuming that Windows-users and Linux-users would be equally inclined to up- or down-vote any given story. That is, the proportion of voters (your "small minority" that votes) is the same in Windows-users population and Linux-users population.
Your point about Haters vs Haters-fighters is well taken.