Windows 7 computers showing up in network as "Media Devices"

Those machines probably have set up as Media Servers by running the corresponding sharing services. Switching those services off will remove their display.

Media Streaming is Off

turn off upnp or dlna

Turn off network discovery. This will not affect file and printer sharing.

UPNP is off.

What will be the noticeable effects of turning off network discovery?  Will the computers still show up in Network?

Network discovery is used if you have no server (in a home, peer-based network). It is not necessary for browsing the network.

OK.  This is a peer-to-peer network.
So, what do you mean "it's not necessary for browsing the network"?
What doesn't work if Network Discovery is disabled?

Name service?
"Network" page not populated?

Network device info like Media Streaming Server, beamer, switch ... will be missing. You still still see the names of Windows PCs.

OK.  I did an experiment.
This is on a Windows 7 Pro system.
I turned off Network Discovery.
Rebooted.
The system disappeared from the "Network" / Computer list. (Not what I wanted)
I believe it was still accessible via UNC and IP address.
Then I turned Network Discovery back on.
Rebooted.
The computer is *still* not visible again in the "Network"'Computer  list after days.....
?????

In the advanced sharing settings does it show if your on a home or work profile? This will be listed on the first dropdown item.

Qlemo:  "it is unreliable" is a very interesting way to put it.  I would have to agree.  Yet, it's what's available and users expect to make use of it.

As background, on this network, none of the Browser services are turned off.
So, I turned off the Computer Browser service on the Master Browser to force a change.
With the new Master Browser, I get this:

results of running nbtstat -a [ipaddress] on two computers:

      Name                              Type         Status
   --------------------------------------------
   SAO                       <00>  UNIQUE      Registered
   WORKGROUP      <00>  GROUP       Registered
   SAO                       <20>  UNIQUE      Registered
   WORKGROUP      <1E>  GROUP       Registered

   Name               Type         Status
 ---------------------------------------------
 FRED_40R             <00>  UNIQUE      Registered
 WORKGROUP      <00>  GROUP       Registered
 FRED_40R             <20>  UNIQUE      Registered

The first computer "SAO" shows up in the Network/Computer list and the second computer FRED_40R does not.
Same result using LANSCAN (which also identifies the Master).

I note that the
WORKGROUP      <1E>  GROUP       Registered
entry only shows up for the first one and not for the second one... so that's a difference but I don't understand

You have to define a network type on the pc.
http://www.dummies.com/how-to/content/how-to-change-a-network-type-in-windows-7.html

I'm surprised turning off network discovery caused this issue.

I assume the pc is behind a server correct? On the pc in question have you run gpupdate /force

All of the computers involved are in workgroup:WORKGROUP and all of their networks are type: Work .. or they would be / should be.  The workstation involved certainly is.
I should have said that this is a peer-to-peer network.

I did discover that this workstation had its Computer Browser service disabled.  I enabled it but don't see any changes, even in the nbtstat results.  Still not on the list of computers.  (Did I say this is Win7 Pro?)

I wonder if this could be a clue:
When I open "Network" on this one workstation (the one that's missing from the list), the list populates very slowly.  Maybe 5 computers of 20-25 show up initially and then the Network "address bar" shows a green bar moving slowly to the right.  Eventually all of the computers are listed (except this one).

I would have expected the "server" service to be running and it is.  Thanks for the suggestion.

The population of the "Network/Computer" list remains to be dreadfully slow.   I've tried Wireshark on this workstation but I'm not quite sure what to look for.  It almost seems like it's polling all the computers on its own.  But I don't see that in the traffic necessarily....

Thanks all!  There were any number of good pointers and ideas.  And, the last one led me to the solution somewhat indirectly.