Can't get into my machine. Password Expired?

After being away from my computer for some time I came
back to find that my password has expired. I try to
enter the old one but it won't let me in and I can't even
get to the desktop to shut down the computer. How do I
get past this and reconfigure the machine so I don't have
this problem again? I tried to search this forum, as I'm
sure I'm not the only one to encounter this, but none of
the posts that I found had a remedy listed. Any help
would be greatly appreciated.