This problem happens when using IE8 to connect to an exchange 2003 OWA instance that isn't fully patched (and you can't get your admin to patch it)
This fix also works for IE8 crashing while using (exchange 2003) OWA when you hit Send on an email
I grabbed the latest S/MIME Controls setupmcl.exe from the KB924334
file version 220.127.116.11 for the executable, when installed its version 6.5.7651.60
Installed that on my Windows 7/ IE8 machine and OWA now works again.