I'm seeing a handful of folks with similar symptoms, but there may well be
multiple possible causes.
One cause of this message in the past has related to non-default settings in
the long and short date formats for the locale. Apparently these can
confuse the date logic in beta1 and make it think it has expired--sometimes
with nonsensical dates listed.
Another cause I can imagine is that the reinstallation/update is not
successfully replacing all the files from the previous install. This is
what I'm inclined to ascribe your problem to.
What version of Windows are you running, and, if XP, is it Home, or
Professional?
Are there multiple Administrator capability accounts?
Can you sign in as an administrator--preferably the same administrator which
originally installed the program--and uninstall Microsoft Antispyware, and
then delete the installation folder and all sub-folders (typically
c:\program files\microsoft antispyware.)
You will lose all customised settings, and access to any quarantined items,
and history.
Then reinstall from the download you made recently.
Let's see if that does the job--if not, we'll need to dig deeper.