Please help! This is driving me nuts!

For some reason that I have not been able to identify, when I try to select a range of adjacent cells using the Ctrl+Shft+Arrow Key combination in the Formula Bar, rather than select the range of cells I would like the formula to operate on, only the first cell in the range is selected!.

Example worksheet range:

Col B
Row 1 10
Row 2 10
Row 3 10
Row 4 10
Row 5 =SUM(

1. I enter =SUM( in cell B5
2. I select the last cell in the range I want to sum by hitting the up arrow - B4
3. I hit Ctrl+Shift+Up Arrow+Enter - Until today, this resulted in =SUM(B1:B4) in cell B5.
4. For some extremely infuriating reason, the resulting formula in Cell B5 = SUM(B4)!!!!!!!!!!!!!!!!!!!!

This happens in all workbooks even when I restart Excel!

Somebody please please tell me why this happens and how I can fix it!

Thanks - Walker