Aha. My date was set to March 2012! The day and time of day were correct but the month was 2 months ahead.
I have NO idea how that happened, and there's no way I did that, so there's something seriously glitchy going on with WinXP, or with something related such as its time server.
Most PC clocks lose time steadily. I use Neutron to correct the system clock on this box as a daily scheduled task. As a result, my date/time is within a second or two of perfect accuracy one hundred percent of the time. So this is a very odd occurrence.
Basic issue is that the certificate is valid in a particular time window. If your PC time is WRONG, then you mistakenly think that the certificate is expired. (All the people who saw the certificate expire early).
Setting your clock back could cause problems now, because the new certificate is not valid prior to today.