I have seen WMP11 in Vista described as "broken". Since MS seems to have no interest in mending it, it's best to regard uPnP as a lost cause in Vista. XBMC is the only reasonably reliable solution anyone's come up with so far. (Personally, I can't live with the interface).
I've also had very little success using WMP as a DLNA server in Vista. I can, however, confirm that Serviio does work and is free. If you try it, be patient - it takes a long time to index a large media collection. Also, like most DLNA servers, it will not serve most HD content. Even .mkv files that will run if imported on to the Humax will not play over DLNA.
Suggestions: for viewing Humax files on a Vista PC, it's easier to decrypt the file by copying it to the virtual disk (using modified firmware) and transfer it to the PC with FTP. Use VLC or Splash Player to view it.
Similarly, to view PC files on the Humax, transfer with FTP, but be prepared for a high failure rate with .mkv files in particular.
For the enthusiast: download LiLi (Live Linux) and an ISO of Linux Mint and create a bootable linux usb stick (it's actually dead easy!). Mint comes complete with VLC and the Linux version of VLC will happily play files from the Humax media server, using uPnP. Don't bother to look for the same functionality in Windows VLC - it's not there.