In addition to the other guidance, here are a couple of more tips:
Your message store may be corrupt. First, make sure you have
installed Vista's SP1 and SP2 updates. If the problem continues,
try running the various repair functions in the WMUtil program:
http://www.oehelp.com/WMUtil
If no improvement, this issue 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
