Laptop says good connection, but no access

I have wireless networks set up in two houses, both using LinkSys Wireless-G systems.
Everything was working fine in both houses until about a week ago.
Now the laptop connects to the Internet at one, but not the other.
The machine says it is connected and the signal is strong, but I can't get/send email or use the internet.
I've tried the "repair" option and nothing changed. I know the DSL connection is good because the desktop which is also on the network works.
OS is XP.
Any ideas?