Update: Claude Henchoz shows an easier way to do this in the comments, check below.
It seems like Apple is only distributing the AirPort Utility on the CD with the unit. This tool is required to manage AirPort Basestations. On their website there are only updates which require a previous version to be installed. So if you’ve lost your CD or if you forget to grab it with you when you loan somebody’s AirPort, you’re screwed.
I found a workaround for this for Mac OS X, but not for Windows. However, these days most ‘updates’ usually contain the entire original application which is also the case with this. There are two ways to fix this. They both start with getting the .MSI file.
Start the installer and get the error “This update requires AirPort version 5.0 to be installed.”. Before clicking the error away, go to your temporary directory (Start -> Run -> %TEMP%). In a subfolder there is a file called ‘APSetup.msi’.
Then there are two ways to continue. The first just gives you the utility to work with. The second patches the .MSI to install properly.
1. Extract the file ‘APUtil.exe’ from this with a tool like Less MSIérables. Open this file and voila, AirPort Utility.
2. Extract scripts.vbs from this tools with ORCA (Tables -> Export Tables -> [x] Binary). Take binary/scripts.vbs.idb and replace
Session.Property(“AP50ISINSTALLED”) = “”
with
Session.Property(“AP50ISINSTALLED”) = “1″
Then put this file back (Tables -> Import Tables -> “Binary.idt”) and save (File -> Save). Then run the APSetup.msi and voila, installer works. There are probably easier ways to fool the installer, but this worked for me. 
Ok after an hour of playing with Orca I figured it out for Windows…
I found out that by deleting 2 keys and just saving the .msi again the upgrade worked.
I did a search for the error and found this:
NoAP50Error 275 This update requires AirPort version 5.0 or greater to be installed.
So then I did a search for NoAP50Error
and found 2 entries, which I simply hit right click/cut and then saved back.
Then the upgrade worked fine!
Thanks
I love you!
Why would Apple not release software whose sole purpose is to manage an Apple WAP? Why are they the only software house that I know of that still thinks a physical CD is anything worth valuing? And yes, why can’t they give the stinking thing a stinking browser interface, like everyone else in the world?
I found the .msi, just deleted one row (“NoAP50Error 275 This update requires AirPort version 5.0 or greater to be installed.”), ran it and all was well.
Thanks!
The easiest way to do this is:
1) Download “AirPort Base Station Update 2007-002 for Windows” from http://www.apple.com/downloads/macosx/apple/application_updates/airportbasestationupdate2007002forwindows.html
2) Use any decent (un-)compression program (7zip works) to extract AirPort521Update.exe
3) Use this command line: msiexec /i APSetup.msi /qb AP50ISINSTALLED=1
4) Be happy.
Thanks Claude, you saved me a bunch of aggravation.
It worked for me in Vista once I navigated to the c:\”USERNAME”\DESKTOP directory,
where I had extracted the files.
Count me happy!