Did you move to Canberra recently? My commiserations
It would be useful to figure out whether the Windows 7 box is trying to reconnect and failing, or whether it's not trying at all. The most direct way to do that would be to run a packet trace (
www.wireshark.org) on the XP target (harder to do it on Windows 7 while it's logging on), and then analyse the resultant trace to establish whether an attempt is being made to reconnect.
If the Windows 7 box is trying and failing, that's relatively easy to troubleshoot because the trace should reveal the nature of the failure. Otherwise, if there's no attempt at all, it becomes necessary to enable additional ("userenv") logging, and as of Vista they've made that a real PITA.
If you do end up generating a trace (it's very simple - like a tape recorder) and you'd like some assistance doing the analysis, just ask.