Norton antivirus is among the top three troublemakers for Windows Mail.
Symptoms often don't surface until several months have gone by, or
until an update occurs. A recent update to Norton allowed only every other
attachment to be opened. Be sure you have the latest Norton update, which
may fix that problem.
Also, be sure to disable Norton's email scanning. If the problem persists,
do a trial uninstall, followed by using their debris removal tool:
http://service1.symantec.com/Support/tsgeninfo.nsf/docid/2005033108162039
Instead of uninstalling your antivirus, another option is to upgrade to
Windows Live Mail, which is more resistant to the adverse effects of overly
intrusive antivirus products:
http://get.live.com/wlmail/overview
In any case you must disable antivirus email scanning, for the reason
explained here:
http://thundercloud.net/infoave/tutorials/email-scanning/index.htm