Network problems ?!?

I hope someone can help....this is drivin' me crazy all day now...ha ha !!!
Maybe its really simple...but I'm using Mac for a few months only...
so its all new to me...

lets start at the beginning...
I had a network, 1 Imac 24" and 2 pc's...all worked great together..
I watched 1080p movies from my imac over the network to my Playon HD (streamer), worked fine...Mac is on, I switched on the Playon HD, started the browser, it found WORKGROUP, and I could see my 2 pc's and my Imac....could stream every file from the Mac and the pc's...

yesterday I bought a new 27" Imac, because one of the pc's was getting a bit old, so I pulled the network cable out and removed the pc, in its place I connected the network cable to my new Imac....so far so good...I could transfer stuff between both Imac's...so everything seemed to work fine...
But when I switched on the Playon HD, it started looking, but couldnt log in on my WORKGROUP....it did not find both Imac's....
I checked if everything was the same on both Imac's (sharing files) and changed a few things...when I tried to log on to the Play on HD, it still couldnt find the WORKGROUP and the Imac's....

I thought maybe it had something to do with the new Imac, so I disconnected the network cable from the new Imac, so the network was back to how it was before (minus the one pc, I changed for the new Imac). I tried again but still couldnt log on....It cant seem to find the WORKGOUP ?!?

can it be that the name WORKGROUP (the network name) is changed ?
I don't know where to find that info...I can see the names of both Imac's, but the name WORKGROUP, is nowhere to be found on both imac's, so I guess that the problem....
Hope this all makes sence........

 

Thanx for the fast reply's...
I will check the Playon HD forum....hope that's the problem