I cannot download the add-in on my laptop with Windows Vista and Office 2007. I have succesfully downloaded the add-in on my PC with Windows XP and Office 2003. On both machines I have the same version of Norton Security. The browser I use is IE.

What could be the problem? 

After posting this problem, I had more problems with my Vista machine, ending in having Vista completely reinstalled. (What a pity it was not possible to have Windows XP installed...). After that it was no problem to download the add in. It works fine now.