There are a few things this could be ... so let's tackle them one-at-a-time.
This round, we'll try the LLTD package ...
While this is supposed to be included in SP3 (and thus won't install on SP3), I've seen several indications that it may not be. But you can "brute force" the install on an SP3 system ...
(a) Download the package [
http://www.microsoft.com/Downloads/details.aspx?familyid=4F01A31D-EE46-481E-BA11-37F485FA34EA&displaylang=en ] ... pay attention to where you save it :-)
(b) Extract the files from the package by running it with the following at the end of the command line: " -x:c:\KB922120"
i.e. open a command window (Start - Run - CMD), navigate to the folder where you saved the file, and run the command "windowsxp-kb922120-v5-x86
-enu.exe -x:c:\KB922120"
(c) You'll get a message that the extraction was completed. Close the command window.
(d) Go to Windows Explorer, go to Tools - Folder Options - View and check the "Show hidden files and folders" button (I'd also uncheck the "Hide extensions for known file types box). You may also want to go to View - Toolbars - Customize and add the "Copy To" option to the toolbar -- this makes it much easier to copy files to a given location without having to drag-and-drop (I REALLY wish they hadn't removed this option from Vista's Explorer). Now navigate to the folder where the files were extracted [ C:\KB922120 if you used my example]. You'll see a folder named SP2QFE. Inside this folder is a file named rspndr.exe ==> copy this file to C:\Windows\System32; then copy rspndr.sys to C:\Windows\System32\Driver
s; and finally copy the file rspndr.inf from the IP subfolder to the C:\Windows\INF folder.
(e) Finally, open another command prompt (Start - Run - CMD), navigate to C:\Windows\System32 (I presume you know how to do this -- just type CD C:\Windows\System32 -- and then type the command "rspndr.exe -I" When you get confirmation that it's done, just close the command window.
NOW check the network in your Vista machine and see if that did the trick. You shouldn't even have to reboot XP ... but if it doesn't work right away, reboot it anyway [never hurts :-) ].
... and of course, if you should ever reload the system, be sure to apply the LLTD package BEFORE you update to SP3 :-)
(To do that you simply double-click on the package you downloaded.)