problems loading/finding/using memoir package/class

I am an author who just wants to typeset my books well for 
print-on-demand publication.  I use a Win XP machine, gvim editor, 
and cygwin environment, but I haven't been able to get the CTAN 
distribution to work.  I tried MiKTeX and got a Hello-world TeX file 
going at once, so MiKTeX seems to be the way to go.  TeXworks has 
read in and output to .pdf the entire 110k word manuscript of my 
novel, so capacity is no problem.  The 'memoir' class seems to be 
well thought of, and a week ago I tried:

\documentclass{scrartcl}
\usepackage{memoir}
\begin{document}
Hello, world! memoir ...
\end{document}

and watched 'memoir' be loaded and the .tex file be compiled by 
pdfLaTeX and output to .pdf.  Yesterday I tried the same thing, but got:

! LaTeX Error: File `memoir.sty' not found.

Type X to quit or <RETURN> to proceed,

or enter new name. (Default extension: sty)

Enter file name:

I fiddled around with the MiKTeX package manager, which did not show 
'memoir' as being installed (!), tried to install 'memoir' (but got 
errors saying, as I recall, "wrong parameter"), tidied up the MS 
registry, managed to fake-uninstall and then re-install 'memoir' 
(according to the package mgr).  I changed 'documentclass' to 'book' 
and added a command specific to 'memoir':

\documentclass{book}
\usepackage{memoir}
\begin{document}
\stockheight = 798pt
Hello, world! memoir ...
\end{document}

and tried again.  I got:

! LaTeX Error: File `memoir.sty' not found.
Type X to quit or <RETURN> to proceed,
or enter new name. (Default extension: sty)
Enter file name:

I entered:  C:/Documents and Settings/usr01/Application 
Data/MiKTeX/2.9/tex/latex/memoir/memoir.cls<ret>

which is where 'find /cygdrive/c -name memoir* -print' (from cygwin 
console) found 'memoir.cls'.  It responded:

! LaTeX Error: Missing \begin{document}.
See the LaTeX manual or LaTeX Companion for explanation.
Type H <return> for immediate help.
...
l.3 \begin
{document}
?

This makes no sense to me.

-- Why doesn't pdfLaTeX find the 'memoir' package?

-- Why on earth doesn't it see the 'begin{document}' line that's right there?

-- What changed between last week and yesterday?
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