What do you mean, is this a problem actually in Excel... or do you mean when
displayed in a windows folder? If the latter you switch between the 2 by
going to <View><Arrange Icons By><Name>. Each time you do this it switches
between a-z and z-a.
You have probably pressed the key combination (I can't remember the exact
one offhand), that reverses the spreadsheet column list for right-to-left
languages such as Hebrew.