Might seem like I just hate MS, but honestly I would be just as upset as a potential investor or partner that they were running Windows in the first place. Licensed or not. Unless of course your startup was focused somehow on Windows development, users, etc. where it would obviously be necessary.
Startups are hard enough as it is, no need to make them harder.
Hmm, if you have an investor that dismisses a platform summarily because of his own biases, then the company is better off without that investor.
The investor should ask "and why are you using that platform?" and then make an evaluation based on the answer. Anything less and you have a pretty bad investor. Best to keep away from such people.
Microsoft Bizspark (http://www.microsoft.com/bizspark/) gives startups that are less than 3 years old and earn less than $1M in annual revenue free MSDN Ultimate access.
That means free access to all of MS software. It's actually a really amazing program.
The BizSpark program has limitations (although still a fantastic deal!).
* AFAIK you are licenced to use the software for development and testing purposes ONLY. Not internal use (see the BizSpark FAQ, even though other BizSpark materials seem to imply otherwise).
* Except you do get a production 2 proc SQL Server licence, and Windows Server licences... However you may not be able to use the licences with some hosting providers (due to their licences with Microsoft, or just unfamiliarity or complexity overhead).
* The BizSpark websites are a time-wasting labyrinth whenever you must update anything (might have improved?).
* There are so many catch-alls to prevent "misuse" of the licence, that it looks as though you can't legally use any of the software... although in spirit you can use it and Microsoft isn't going to chase you... (at least that was how I read the licences last time I looked but IANAL YMMV).
Startups are hard enough as it is, no need to make them harder.