If I open a CSV or Paste in certian types of text data, Excel will reformat
it based on what it thinks the data really is. I would like it to leave my
data alone unless I tell it to modify something.

For exmaple, if I type in notepad the text 1/23, and copy this and paste it
into a new spreadsheet, the result is 23-Jan ( depending on your date
settings. ) Why does it assume this value is a date, and how do I tell it to
stop?

Another example is data that looks like a number that has one or more
leading 0's. Excl seems to feel the need to strip the leading 0's to make the
value more "numberish" I guess. I don't see why it has to assume that a value
is a number just because it contains all digits, it could just as easily
belong to a set of serial numbers that occasionally have characters as well.