Lightroom 4.2 install fails at setup64.exe

Environment:

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- OS: Windows XP fully patched.

- Running as an admin user.

- Old lightroom version: from 2007, uninstalled a couple of years ago, incl. Lighroom folder in My Pictures.

- ZoneAlarm logs showing noting relevant.

Procedure to problem:

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I've downloaded the trial; the file (Lightroom_4_LS11.exe) is about the right size (769,437KB).

Double-clicking it starts OK.

The first dialog box allows me to specify the location for the working folder (makes no difference if I take the default or specify another one; there's 20Gb free).

Next is a progress box: it gets to 100%, last working on setup64.exe, as below

At this stage a few files are in the working folder, including setup64.exe but with size 0.

(setup32.exe was OK)

Then immdiately this:

(That is: "A problem occurred while extracting some files. Check available space on your computer and write privileges on the destination folder.")

I see other folks (on different fora) have been getting this last dialog box, with other Adobe products (eg. CS6), but I have yet to find a solution that works.

Re-downloading didn't help.

Please help because I'd really like to try LR, though obviously this experience doesn't fill me with confidence!

Mike