So it's just the connection for downloads that's the problem...your LW's connected to the network ok???

If you're sure that it's not fake files that you're seeing
Fake files showing up in search results
and if absolutely everything you try to download gives you the connecting/need more sources message, delete your preferences folder when LW's properly closed.

See point 3 below (also point 2 if you've used much older versions of LW in the past).
Fixes for LimeWire
This will kill all of your settings within options, you'll have to go back through later & reset things to the way you like them.
You'll have to show hidden files & folders to see that preferences folder, if you're not sure of how to do that within Vista, check out this sticky.
LimeWire Pref Folder in Vista

When you set up port forwarding, did you make sure to set up a static IP address first? Also remember that port forwarded within the router, LW's listening port & port within the 'manual port forward' box must all be the same number.