Mezzmo and TVersity, as MarkHotchkiss mentioned, are a couple that first came to mind.
There is also Serviio, Twonky, FUPPES and several others out there in the wild (just search google for DLNA server).
I think if you are planning to use these with media boxes (WD, Roku, Patriot, etc.) you will be fine. You will run into issues (like I did) when trying to use these in conjunction with Blu-Ray Players and HDTV's that can access DLNA servers.
I tried running these DLNA media servers with my BD player (Samsung BD-C6500) and my HDTV's, trying to stream BD quality MKV's over a 100mbps LAN. The reason for this was that my BD player and TV didn't seem to have the processing power to handle the MKV's running off the standard Windows DLNA serving capabilities. I was also running into issue with the DLNA media software because there is a very intricate system for defining the devices you are using with these software titles (they all have built in profiles for different devices out in the wild) as each device has different (and very limited) codec support.
I don't think you will have any issue running these if you have a media player, heck they might provide better playback than the standard DLNA server with Windows.