"Network slows down with RR Cable modem connected"

OK, I'm baffled. My network slows down when a RoadRunner cable modem is connected to it. Here's the configuration:
1 5-port 10/100BaseT switch
1 PC with 10/100BaseT Ethernet card connected to switch port.
1 Mac with 10/100BaseT Ethernet connected to switch port.
1 Streaming media player (100BaseT) connected to switch port

With that configuration, I get full bandwidth between the PC and the streaming media player, i.e. MPEG movies play fine.

Next, I connect the Roadrunner cable modem to the 'Uplink' port of the switch. The cable modem is a 10BaseT device, and the switch shows the cable modem at that speed, and the PC still at 100BaseT speed.

Now, streaming video won't play - jitters and halts, and music starts slowly...all indications of insufficient bandwith.

QUESTION: What could connecting the Roadrunner modem have to do with anything? The modem works fine, my internet connection through the PC is great, but somehow the speed of connection from the media player to the PC is affected by connecting the modem.

HELP!!!

Try placing a router between the switch and the rr modem. This will take the rr modem of the broadcast domain and place it on its own domain. There is also something else you can try before buying the router. This is for the pc's that are running windows and only if they are running xp,2000. Right click my computer, goto manage, look for services, go into services, locate messenger service, stop and disable this service. If that doesnt work get a router. There is also another reason to get the router. A switch does not have DHCP and therefore is unable to assign ip address. This will get rr attention and cost you additional money a month for 2 ip address to your location that are not supported. The service you have gives you 1 cable modem and access for one computer directly connected to that modem. The other computer and media server are not on that package