This is how I feel... When I first started my NA job I only knew so much. I installed Apache, PHP, and MySQL so I could run an Intranet designed using PHP (and other various reasons).
After a year I asked myself why I was doing that when IIS can do every thing Apache can and ASP can do every thing PHP can.
My advice to you is to stick with IIS and ASP for a Windows network. Don't complicate your situation. Installing Apache, PHP, and MySQL just give you more to maintain especially when IIS keeps getting better with each release.