That error 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.
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