copy it pops up a box titled "Microsoft Excel" that says "The
picture is too large and will be truncated."

For example, right now if I start up Excel it defaults to a blank
worksheet for "Book1." I click on the "A" column to select it, CTRL-C
to copy, and the error box pops up.  I click [Ok] and it pops up a
second time.

Again on this blank brand new sheet if I select cells A1 to A74 (one
column, 74 cells), I can do the CRTL-C (or Edit / Copy) and there's no
pop-up.  If I changed the selection to end at A75 (75 cells) I get the
pop-up.

In the horizontal direction I get the same issue if I have 19 or more
cells selected.  Thus I can select an 18 column by 74 row region
without a pop-up but as soon as I have more than 18 columns or 74 rows
I get the pop-up.

Copy/pasting on the test sheet seems to work regardless of the error
but I don't want to chance that on my real data.

I'm aware of http://support.microsoft.com/?kbid=318265 but I'm not
trying to copy as a picture. As a test I held the shift key down,
clicked on "Edit" and see that I'm offered "Copy Picture..."
rather than the usual "Copy Ctrl-C" and further with the shift key
down and selecting "Copy Picture..." Excel pops up a Copy Picture
dialog box and in fact seems happy with creating a 1000 row by 26
column "Picture" though on pasting it turned out I got a 72 row by
19 column image. I really don't care that the picture did not have
all of my data as I've never used that option.

This happens a couple of times a year to me. In the past I had been
able to fix it by shutting down Excel and restarting it but this time
Excel seems determined to stay in this mode and even a reboot did not
help.

Marc