Excel password mechanism failure

I have a 2 PC Home/Biz network. The older PC runs Win XP and the new PC runs
Win 7. I have a password protected Excel 2007 document on both systems. (I
copied the file from the XP system to the WIN 7 system in January when I
installed the WIN 7 system.) I open the file 3-4 times a month on the WIN 7
system. Today I cannot access the WIN 7 stored file using the WIN 7 PC or
the XP stored file using the XP PC. On both systems Excel informs me that my
password is incorrect . Simply not possible. I wrote the password down and
have been regularly using it since Jan 2010. The only thing common to both
systems are Windows updates. Has an update hosed the password mechanism in
Excel 2007? An infection of some sort?
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bcd