Download and install this fix, with Windows Mail not running:
http://support.microsoft.com/default.aspx/kb/941090
Although it's labelled as for mail stuck in the Outbox, it also
works in other folders. If you don't know whether to use
the 32-bit version or the 64-bit version, try the 32-bit
version first. Some common antivirus programs, such as
most of those from Norton or McAfee, ot a recent one
from Trend, often cause such problems, although seldom
immediately, so if you have such a program, remember
where to find the fix - you may need it again.