I use Serviio because I don't much care for Windows Media Player, plus I think a few more formats are supported. Serviio is free btw. In essence, however, all DLNA servers should accomplish the same thing, some may just support a few more filetypes.
There is no native app as far as I'm aware on the N7. Just tested out aVia and MediaHouse. Both work perfectly fine streaming from my DLNA server. Both are free but they have pro (read: paid) versions. If you can tolerate ads, the free version may be perfectly acceptable. If not, the N7 still comes with a $25 play credit so you could easily pick up aVia for 99 cents and not have to pay out of pocket.
If they can pick up serviio, I'm certain they'll pick up Windows Media Player 12 served files as well. Also, if you bought the Nexus at a local store, most have a limited return window where you could take it back for a refund if it doesn't stream as you're hoping.
Edit: scratch that, aVia's paid version is just for more languages. I didn't test it out for too long; I just wanted to see if it would work fine.