MS Outlook freezes when trying to a attach a file. Delay sometimes can take as long as couple of minutes. Windows Explorer can also freeze at the same time.
This behaviour is most often caused by a Disconnected Network Drive or My Network Places shortcut pointing to non existing network location. Simply disconnect the old drive or delete offending shortcut and problem should be fixed.
If this doesn't help you can try deleting Temporary Internet Files and clearing Internet Explorer History.
MS Outlook 2003
MS Outlook 2007
MS Outlook 2010
