Alice Faber answered this one for me in a usenet posting:
If you're connecting through the Windows XP interface, click on the
Advanced tab. In some XP installations, there's a default setting that
will guarantee that you won't connect, hidden on the Advanced tab. I
forget the exact wording of it (I don't have access to an XP box to
check), but it will say something about smart card authentication.
On the main tab, you may have to search out a pulldown menu that will
allow you to selecte Shared authentication also.
The key phrase is "shared authentication". A google search on this phrase and "airport" brought a lot of hits:
A few key notes:
A. http://www.opentechsupport.net/forums/archive/topic/18986-1.html
Be sure the right XP services are running
1.) Go to Start -> Control Panel -> Performance and Maintenance -> Administrative Tools -> Services.
2.) Scroll down to Wireless Zero Configuration, and double-click Wireless Zero Configuration.
3.) On the General tab, after Startup type, change the startup type to Automatic.
4.) Start the service by clicking Start.
B. http://stevenf.com/mt/archives/000471.php
A few discussions, and an aside that Apple's advanced tech support was clueless.
This is where they point out the need to enable "shared authentication". There's a hint that older 802.11b implementations don't support it.
C. http://wireless-starter-kit.com/airportblog/
Lots of discussions on Airport wireless.
D. http://www.macwireless.com/index.html
Serious Mac wireless vendor.
E. http://discussions.info.apple.com/WebX?13@83.CGwtaOvycER.930@.688ec533/0
Accessing a Airport Network with a Windows XP PC or laptop (WEP enabled)
http://www.ifelix.co.uk/tech/1000.html
Patch required if running Windows XP and WPA on an AEBS
http://www.ifelix.co.uk/tech/2003.html
The three main reasons people have problems are
1. WEP Key in plain text and not HEX
If you enable WEP on the AEBS, you need to note down the Hex Airport Network Password, if you are using 128bit encryption this will be a 26 digit number.
If you open the Airport Admin Utility you should see something like this
http://www.ifelix.co.uk/tech/images004/ae005.gif
You may need to click a button called Password
Note that I have blanked out my passwords
The Wireless ASCII Equivalent Password is the plain text network password you use to access your Airport network with a Mac.
The Wireless Hex Equivalent Password is the hex version of the plain text, and this is what you would use with your PC.
2.IEEE 802.1x authentication is on
Ensure that the Enable IEEE 802.1x authentication for this network check box is not checked.
3. Authentication Mode is not set to Shared
Ensure Network Authentication (Shared Mode) box is ticked
On a separate topic, a wireless bridge for 802.11g AEBS networks, I found:
http://www.macwireless.com/html/products/11g/11gEthAdapter.html
and an other great usenet posting sent me to:
http://www.buffalotech.com/wireless/products/airstation/WLAG54C.html
Bottom line: usenet lives.