1. If you are using McAffee firewall, then make an allowance for LW but disable Vista's firewall;
Setting up an exception for LimeWire in your software firewall (click in link)
2. If multiple computers are used on same LAN network, then each MUST use a different connection & listening port.

Go to tools > options > advanced > super really advanced > Firewalls & make sure both connection & listening ports are different from each of the other LW's running on the other computers.

If not, change them to a port between 49152 - 65535. For example, port 64049 sounds like a good one.
3. 'Remove' the gnutella.net file from the preferences folder. It appears for LW 5 it is best removing it rather than replacing it.

LW will create a new one itself.
4. Follow these tips
If LW Will Not Connect (click on the blue link) about disabling Performance.
5. If all is still no good, then perhaps await the next LW 5 update since the programmers are trying to solve a few problems. In meantime you can go back to LW 4.18. If problems with that version then see suggestions in no. #4 above.
* Which model Speedstream are you using? You are able to give a special 'pass' to specific programs in Speedstream NAT/firewall rules.