[clearcomms]

My wife's laptop connects to the internet via a wireless connection to my router. This has worked perfectly for months, till the last few days.
Now there are repeated problems. Earlier this morning, for instance, she accessed her e-mails and got good results on an ADSL speed test.
But at this moment Thunderbird can't access the server, and neither can Firefox. They don't appears to have a network connection.
Yet Windows shows the network as connected, signal strength very good, Speed 54 Mbps.
I've tried rebooting, and disconnecting and reconnecting.
It isn't her firewall (Zonealarm Security Suite) causing it, as I experimentally shut that down, without effect.
She runs XP Home, fully updated.
All suggestions gratefully received, as this is driving us both crazy!