If you're using a router, then you should check the manufacturers web site to see if there is an updated firmware version at their site. Most sites have detailed instructions on upgrading your firmware. If you don't know where to go from that, please post back with your router make and model, and I'll see if I can dig a link and instructions up for you.
If you're not using a router, and you're on broadband, try resetting your cable/dsl modem. Unplug everything from the back of the modem for 3 minutes. Plug things back in in this order:
Power cord
Feed from cable/dsl company
Network cable to router/computer/hub/switch
If you need even more detailed instructions, that's fine, but we need to know more about your computer and your network. Are you on PS2 or PC? What specs does your computer have? To find out, you can go to:
Start >> Programs >> PlayOnline >> PlayOnline Viewer >> PlayOnline Viewer Config
Then click the System Information button. Copy and paste everything you see under the System tab.