We have 3 PCs at home, 2 with XP on them and 1 with Vista. We use BT
Wireless Broadband. Every few days the Vista PC will lose connection -
it is still receiving the wireless signal from the router, but it
cannot get to the internet.

This rarely, if ever, happens on the XP machines.

The only way to re-connect the Vista machine to the internet is to
power off/on the router and then it reconnects without a problem.

When it is connected the IP address on the Vista laptop is
192.168.1.65.
However when it has disconnected its IP address becomes 169.254.96.107

I followed advice from another board where the suggestion was made
that Vista was being very picky about getting it's DHCP settings. XP
used to wait 3mins for a DHCP response but Vista only waits 6 seconds.
So anything that delays the DHCP server - ie the router- from
responding within 6 seconds will cause Vista to assign an APIPA
address.

So I unchecked IPv6 in the properties for the Wireless Connection and
then amended Properties for IPv4 so it obtains IP Address
Automatically and has a User Configured Alternate Configuration as
follows:
IP Address: 192.168.2.200
Subnet Mask: 255.255.255.0
Default Gateway: 192.168.2.1
Preferred DNS: 192.168.2.1

This has not stopped the problem from occurring. Every couple of days
or so the Vista machine loses internet connection and the only way to
get it back is to power off and on the router.

Anybody got any ideas?