Trouble Connecting to Airport Express network: "Out of Range" Erro

I'm having trouble getting my HP laptop to connect to my wireless network
(using an Airport Express router). My Macintosh Powerbook connects fine to
the network, and I've connected another PC laptop to the same network before
with no problems (and with no extra configuration). Both computers had
Windows XP Pro (and SP2).

The HP laptop can see the network, but when I attempt to connect I get an
error message:
"Windows is unable to connect to the selected network. The network may no
longer be in range. Please refresh the list of available networks, and try to
connect again."

When I get this message, I'm right next to the Airport Express, and I'm
pretty sure it's not actually out of range. I've disabled the firewall on the
PC, removed all wireless security, run Windows Update, and still am not able
to connect. I've also tried using the Airport Express software, but the
computer doesn't see the airport express to configure it. I am able to
connect to the neighbor's wifi, so I know the computer's wireless access is
intact.

Has anyone else experienced a similar issue and found a way to work around it?

As an update to my post, per a workaround recommended by Microsoft, I entered
the Network Key by hand in the properties option. Now when I try to connect
to my network, it asks for the network key, and times out trying to connect.

Still stumped. Any thoughts?