If you are doing ASP.NET and C#. I would advise to stick with Windows. The main reason I see would be Visual Studio. You could also run it in a virtual machine if you want.
Althrough I never used it or wrote any line of C#, I know Novell says in big letters on its website it supports very well ASP.NET and Microsoft solutions.
There are some very big companies using Microsoft technologies on UNIX / Linux.
Don't use Adobe technologies (like Flex). They are costy, not portable, limited to 1.5 architecture, proprietary, ...
But aside from Microsoft and Adobe, for all other languages and especially as you're interested in Open Source, you will find Linux to be much better for development. (At least that's what I think, YMMV)
Your development environment will also be much closer if not the same than the production one and this is a big advantage.
You will get the best command line related tools available, which is heavily used in modern frameworks like Ruby on Rails, django, symfony, CakePHP, etc.
If you're planning to write Java code, you can have most of the tools, e.g. IntelliJ IDEA, NetBeans, Eclipse.
Finally, server software is so easy to install with package management (for example on Debian or Ubuntu).
"apt-get install apache2 postgresql-8.4 php5 ..." and you get your LAMP, LAPP, you name it environment without opening any configuration file.)
For everyday / average user tasks, you will find OpenOffice / LibreOffice (now in Ubuntu but it's the same thing without the Oracle logo), Firefox, Thunderbird (I can give you hints how to use it in a MS Exchange environment). They're all open source, complete and convenient. I find them more user-friendly than MS Office and Outlook.
So yes, go the open source / free software road. You won't regret it and will probably never look back.
(However in my daily work, I use Debian with a Windows 7 virtual machine because, even when writing Java, you need to test it out on Windows, especially Internet Explorer that behave very differently from all the other browsers... Safari, Firefox, Opera, Chrome)