Q: Cannot install iTunes v11.0.4 upgrade

I'm having continued trouble with the latest Update to iTunes and Quicktime.  I'm running iTunes v11.0.2.26 on a Windows XP (SP3) machine, and trying to follow the current offers to update to v11.0.4.

If I click the Install link, I get the message:

<Errors occurred while installing the updates.  If the problem persists, choose Tools>Download Only and try installing manually.>

I do this, and it sets off until I receive the message:

<Checking for new software -- server could not be found.  Check your internet settings and try again.>

My setting all seem to be AOK, and in any case this is the ONLY case where I get such a message -- iTunes itself connects to Store and Podcasts without any trouble.

If I try to force an Upgrade through the iTunes menus, I get the message:

<Quicktime has invalid signature.  The Download has been removed>, and then a similar message for iTunes.

Previous Upgrades have been painless -- what's wrong with this one?

Regards,

GRAEME, Yorkshire