Script error when URL button in CHM clicked on some machines

[RiverMonster]

My machine: Windows 7 Enterprise SP1, IE 9.0.8....

The Problem:
i. I made a very simple test HTML topic with this code in the body:
<input type=button onClick="parent.location='http://www.bbc.co.uk/news/'" value='Click here'>
ii. I generate a one topic CHM from this (no errors on generating)
iii. I open the CHM and click the button
Result > The BBC web page opens inside the CHM window as expected

Now I send the CHM to a colleague. Like me she has Windows 7 Enterprise, but she has IE8 not IE9.
She puts the CHM in a local folder, clicks the button and gets the following err msg:

script error.png

I tried on a second machine that runs XP Pro SP3 and IE8 and the err msg also gets shown. So I thought this must be due to IE8. However, I then tried on a third machine running XP Pro SP3 and IE8 and the err msg is not shown.

So it seems that whether or not an err msg is shown is not related to the OS or the IE version.

On the machines where we get the err msg we tried doing this with just the HTML file itself rather than the CHM (double-clicking it to open it in IE and then clicking the button) and it opens the BBC web page without any errors. So the problem seems to be specific to CHM files.

At this point I thought it could be related to the same issue that prevents accessing a CHM across a network so I checked the itssrestrictions>maxallowedzone setting in the registry on the problem machines, but they are set to "4" which should basically allow everything.

So now I am out of ideas. Anyone know what could be causing the err msg?

Thanks,

Adrian