I have been trying and trying to get Qgis Server to working on my Windows 7 machine with no luck. I have read every thread out there on the web related to how to install and nothing works. I'm beginning to think that it's all just a nasty joke that someone is trying to play and that there isn't really a Server available. If anyone has any step-by-step directions that actually work, would love to hear.
I ran into the same problem using the old 32bit OSGeo4w installer from september 2013.
Apache installs without error logs, but the service throws only 500-Errors. alternatively, it downloads the qgis.mapserv.fcgi.exe instead of executing it.
After searching around I read about missing libfcgi.dll causing such problems.
The advanced installer of OSgeo4w offers a fcgi library, but that is not selected as dependency when choosing the qgis-server package. Alternatively I took the driver from my running ms4w installation.
The new 64bit installer of OSGeo4w has included the package fcgi as dependency, but is missing the whole apache package (up to now). So that's not working at all. By the way, qgis-server does not include qgis desktop as dependency, but will not run without that, I assume.
So the 32-bit installer is the only one that might run as QGIS server at the moment.
As per QGIS home page Qgis server actually exists. You can try the procedure mentioned by 'underdark' in the following link.
You may be aware 'underdark' is a moderator of gis.stockexchange.com So the procedure can't go wrong. All the best.