Please help, this is my problem. Some formulas don't work when I type them in.

For example, when I type this formula, it will say there is an error.
"=SUM(1,2,3)"
That does not work, it says invalid format.
but if I do a formula with a range (instead of commas), it works. Such as
"=SUM(A5:C5)" - that works just fine.

I am guessing this has something to do with the commas in the formula. When
I start to type a formula, a formula helper thing will be displayed that is
showing the format of the formula. This format example does not include
commas. It uses spaces instead. So I tried to type the formula with spaces
(instead of commas) and that doesn't work either.

I consider myself a advanced user of excel, I have extensive experience with
excel formulas, so this is really bothering me (especially since I can't type
many formulas because they need commas and that doesn't work).

One more thing. I have existing spreadsheets that I am using (I have created
these months ago). On these, I have formulas that contain commas and they all
work. But if I double-click on the cell (as if I am going to edit the
formula, but I don't edit it, I keep it the same) and press enter, the same
error appears. If I hit escape, it cancels the formula edit request and goes
back to how it was.

I did try to uninstall and reinstall excel, that didn't work. But I think it
is still pulling my info from somewhere.

Thanks for all of your help.