This is not a group for normal Excel features, but rather it is for
macros in the proramming language that is called Visual Basic for
Applications. However, look in the Menu > Tools > Edit. The "Allow
Drag and Drop" should be checked.
On my Excel 97, un-checking it takes away my little box on the lower
right of a selected cell  that allows the feature  you are talking
about. I figured this out by simply looking around a bit. When
something stops, don't forget to look in the preferences - in this
case called options.

thanks for the info. I've been using Excel for quite awhile and
still don't know all the cavets of its use. Didn't know that you could
turn it off and on. I read all these posts just to learn something I
didn't already know.

Neither did I until I poked the Menu and saw the little square
vanish. I thought it was called "auto-fill". It's murder getting rid
of something that spontaneously appears (disapears) and you don't know
what it is called to revert it.
Then there's those things you swear you could change (in an earler
version) and can't now.