I'm sorry to say Claire but there seems to be an issue with the lastest LW that no one will seem to just admit to They just post 'fixes' and get ratty with people who don't seem as technically competant as them. The only thing I can suggest is to try another P2P program or an older version of LW and keep an eye on LW until they've fixed the current release. One minute it was working fine, the next it's stopped for 000's of people. All we get is, "check your firewall", "delete this file", "change this setting" etc.
When I first used L/W it worked with no issues, now I'm being told to try all these different things to fix an issue that was never there before....why release an upgrade that doesn't work?
Sorry, I know this doesn't help much but after reading and trying several 'fixes' over the months this is the only conclusion I can come to