A recent version of the Trend antivirus software (details not
available) causes assorted problems in Windows Mail,
and has to be uninstalled, not just disabled, to stop it from
causing these problems. You sometimes then need to
remove your email account under Windows Mail, then
restart Windows Mail and create the account again.
If this cures your problem, you may want a replacement
antivirus program that doesn't cause such problems.
I recommend either avast! or AVG, either one with a
custom install that tells it to leave out the part that scans
email.
If you mean Windows Vista updates, I installed the
same updates this morning and had no problem
sending myself a message afterwards.