That problem can be caused by interference from an antivirus program.
Which antivirus are you running?
As a minimum, email scanning in the antivirus should be turned off,
although that may not be sufficient to eliminate all bad effects.
In a worst case scenario, your antivirus may need to be uninstalled.
Email scanning in any antivirus should be disabled, for reasons
explained here:
http://thundercloud.net/infoave/tutorials/email-scanning/index.htm
If no improvement after disabling email scanning, consider upgrading
to Windows Live Mail, because it is less prone to suffering bad effects
from overly intrusive antivirus programs:
http://download.live.com/wlmail
