WindowsUpdate V5 on XPSP1 broken

Tried running Windows Update today, and it wanted to install a new
version (V5) of Windows Update, so I said OK. During installation of v5
I got an "access is denied" scripting error, even though I added the
v5 site to Trusted Sites before accepting the update.

Now any attempt to launch WindowsUpdate gets a typically cryptic
Microsoft error message:

Windows Update has encountered an error and cannot display the requested page.
Error number: 0x80072F78

That's all there is, apart from some links to various MS help pages.

Suggestions?

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