There's no mistake. Hiring a programmer is a composite image of many
things good and bad. For 37signals, it's definitely a disadvantage to
still be a Windows user. I wouldn't say that it's impossible, just
considerably harder, to convince us that it didn't matter too much.

The choices you make as a programmer serves as indicators for your
cultural standing and performance. The kind of books you read, the
methodologies that you favor, the pastime projects you're involved
with, and yes, your choice of programming language and computing
environment.

Just like hiring someone with a declared love for Java wouldn't make
sense for 37signals, hiring someone who thinks that Windows is the best
platform for open source use and development doesn't make that much
sense either.

If your funds are tight, I'd see it natural that you picked a free
alternative, like Linux.

Naturally, this is a fairly context-dependent recommendation. If your
dream job is working on C# using Visual Studio for some Microsoft shop,
then of course a Windows setup is a good pick. I'm going to be a bit
baffled as to why that is, but I won't berate your choice of
environment to pursue that dream.

On the other hand, if you want to work with open source technologies
like the Rails stack of Apache/lighttpd, MySQL/PostgreSQL, Ruby/Rails,
etc, I find a strong disconnect with doing so from Windows. It's just
not a natural fit neither from a technological, cultural, or political
perspective. Actively pursuing or celebrating this unnatural fit raises
a red flag for me.