Been banging my head against the wall for a few days now, so I figured I might as well throw it out here for the experts. It's mainly to satisfy my curiosity because I can't quite figure out what's going on.
I was given an old system (Dell Dimension 3000) that would be needed for a family who just needed a basic computer. I wiped the hard drive and installed a clean install of XP Pro SP3 (Dell OEM disk). Everything seemed to go fine, until I tried to access Windows Update for the newest updates. I went from having the site refuse to load at all to hanging at "Checking for the latest updates...". I followed all the usual MS troubleshooting, resetting updates, clearing update cache, etc. Even forcibly reinstalled the Windows Update Agent to no avail. I went as far as reinstalling with full retail media (again XP Pro SP3) and had the same problem.
Finding this to be strange, I installed the same copy of Windows XP in a virtual machine on my main system and ran into exactly the same problem. I've installed XP with SP3 a hundred times and never had trouble with Windows Update.
The best part is, I went ahead and downloaded Autopatcher and downloaded all the post-SP3 updates as of Aug. 28. After they install, with the exception of Windows Genuine Advantage Notifications (which hangs), they install just fine, and now Windows Update works.
Looking through Autopatcher, there really are no updates relevant to Windows Update that would suddenly fix whatever problem is going on with Windows Update. In the past if there was an update needed to make the WU site work, it would explain that and offer the download. I've replicated this on two PCs and one virtual machine.
I don't mind doing a clean install and then using Autopatcher, but it's killing me to know why that seems to be the only thing that fixes Windows Update. Anybody notice this behavior, or have any thoughts?