Whilst it is true that the 32-bit version of IE is the default, it is possible to change it so that the 64-bit version opens instead when the appropriate key is pressed on the keyboard (assuming, of course, that the keys can be reprogrammed). This does not affect the default association from other programs, i.e. email clients.
Open the keyboard configuration program and select the Key Settings
tab (this is the IntelliType
Select (highlight) Web/Home
and click Configure...
Click on Start a program, Web page, or file
and then click Next
Either browse to, or type into the box, one of the following options: 64-bit C:\Program Files\Internet Explorer\iexplore.exe 32-bit
(this is the default if you select Starts Web browser
above) C:\Program Files (x86)\Internet Explorer\iexplore.exe
Finally, click Finish
to close the dialogue panels. When you next press the appropriate key on your keyboard, it will open the selected version of IE.