Windows 7 does not (always) like IE9

A couple of days ago a neighbor asked if I could have a look at his slightly misbehaving PC; links in emails did not open correctly. He was on an all Microsoft setup, with Windows Seven, IE9 and Outlook. The error was as harsh as it was vague;

According to “wonderfully wise web” that problem is typically caused by a lack of a default browser in Windows 7, but given the fact that IE9 was the only browser installed, that did not make a lot of sense.
I was pretty confused, checking and re-applying settings while searching the web, but then I found a forum thread in which someone replied that the 64-bits version of IE9 just cannot be set as default browser, ever. Must be that Windows 7 does not really like 64-bits IE9? I did not find an easy way to switch to 32-bits IE9 and as the neighbor did not object, so I installed the most recent stable version of Firefox on his PC and set that as default browser and all was well. IE9 might not be a bad browser, but sometimes even Windows 7 wants you to install an alternative and who are we to disagree?