How do I change the DEFAULT delimiter for pasting text data?
 

I'd been using another guy's workbook for creating a report, pasting a large
block of data cut from a UNIX system into the spreadsheet. The spreadsheet
defaulted to fixed-width delimiters, and everything worked fine. There
happened to be another spreadsheet in the workbook that required a similar
operation, but delimited by spaces; in this case, a wizard popped up, and I
was able to select "Delimited" rather than "Fixed Width," and then check the
"spaces" block on the next pop-up that listed possible delimiters.

Then one day, without any specific intent or action on my part, the default
behavior of the first spreadsheet changed. Where it used to break the pasted
content on a fixed width pattern, it now delimited by spaces.

I have several questions (asked between muttered curses regarding the insane
proclivity of Microsoft products to do everything for you without telling you
where, how, or why it's doing it, and without providing a consistent
interface to change its behavior):

1) How do I change the default behavior of the paste operation in first
spreadsheet back to what it was?

2) WHY did simply selecting "Delimited" over "Fixed Width" on the wizard,
while pasting into another spreadsheet in the same workbook, change the
default behavior of the paste operation in the first spreadsheet?

3) Why did selecting "Delimited" over "Fixed Width" on the wizard NOT change
said default behavior the first 20 times I did it?

Thanks.