Internet Explorer defaults to 64-bit version

Try editing the registry. Make sure to open the 64-bit registry editor. (you have to close the 32-bit one first).

Go to the registry key HKEY_LOCAL_MACHINE\SOFTWARE\Classes\http\shell\open\command and set the (Default) value to the path (from the 64-bit point of view) of 32-bit IE, which might be "C:\Program Files(x86)\Internet Explorer\IEXPLORE.EXE" -nohome (the "-nohome" is a commandline option which my copy of IE8 adds to this value)

You might want to do the same for the classes https and even htmlfile if you want to double-click HTML files and open them in 32-bit IE.

All this won't help if a 64-bit application executes iexplore.exe directly instead of launching a URL. (in fact I haven't tested my suggestion at all because I don't have 64-bit Windows. Nevertheless it might work)

try changing the defaults in:

control panel > programs > default programs > set associations