Paging the WiFi-isti

I've just got home from setting up a computer belonging to a colleague
again.

It involved formatting the Hard Drive and reinstalling Win XP plus
their Linksys USB WiFi device.

Everything is working fine apart from the WiFi. I've set up quite a
few in my time but this one has me stumped. The software is installed
and it's running but the Wireless connection doesn't work all the
time.

It finds the wireless network which has excellent signal strength and
quality but every few minutes the Linksys tool says it's connected to
the wireless access device but not connected to the internet. The IP
address falls off (I've tried manually assigning one or just using the
auto-assigned one but that doesn't help) and it says it's connected to
the network but not to the internet.

Oddly I downloaded the 200+Mb version of Service Pack 2 and it keps
saying it was connected and working properly and then a few minutes
later that it's connected to the network, but not the internet. The
download continued throughout, even when it said it wasn't connected
to the internet.

I'm not used to Linksys kit, and the Linksys client I downloaded was
quite simple to use. WEP/PSK is set up and working properly and there
is another laptop in the house connected to it which has no problems
whatsoever.

The DHCP lease is set to 0, which it says is 24 hours so I'm stumped.
Any pointers on why the IP keeps falling off, even with excellent
signal quality?

This could happen if you are using the same channel as your neighbours.
Channels 1 and 11 are very popular because they are defaults on many
routers and in residential areas they can get quite crowded. Set your
router to some less crowded channel (you are likely to have a utility
somewhere showing you the other networks and which channels they use).

The router was on channel 11 and although I can't remember seeing what
channels other WiFi networks were on in the local area there were four
or five of them. There's an option to change the channel in the
router. I'll suggest that she changes that.

remember the channels overlap, so only 1, 6 and 11 are actually using
unique frequencies