Issue solved. It was a router problem. It appears that some routers don't support the 'Quality of service' option ( Configure> Settings> Network ; have 'Advanced' ticked) so I just had to turn that off.
On my home computer (the only pc with that issue) I'm running Vista and I'm connected to a relatively cheap Cisco router (can't remember the model and can't check atm).
I never had the issue on my laptop at campus (Win7 connected through VPN).
It seems that it's a very rare issue as the only search results for the error were this thread and an unanswered thread on another forum.
Oh right - cool. Good to know I guess. I asked about Win 7 as the only thing I can find that reliably causes that issue is when Win 7 (that uses IPV6) has a tantrum with Mumble (that uses IPV4) and refuses to play nice. Its not a constant issue is that thing so its hard to fix and flatly should not have caused any problems in the first place.
Anyway, glad you got it sorted.