Windows XP SP3 wsus client not updating

When you run the WSUS Client Diagnostics Tool, you get the following error at the end :

WinHttpDownloadFileToMemory(szURLDest, NULL, 0, NULL, NULL, NULL, &downloadBuffe r) failed with hr=0x80072efd

A connection with the server could not be established

In addition, your WSUS server is most likely not configured to listen on port 80.

In my case manually updating the WUAgent to v7.4 (x86 | x64) solved the problem.

I suspect the client is trying to execute a selfupdate but this fails because the WSUS server is not responding on port 80.