EDIT: updated code because old code overwrote values in cells. This code just changes the font color to white so that you won't see the values if the cells' background is white.
This is not possible just using a formula in Excel. You need to write code in Excel VBA. Here's the steps you need to follow:
1. Set macro security to medium so that you can run this code.
Tools->macro->security
Choose medium. This will let you choose whether or not to enable macros (programs) whenever opening an Excel file containing them. Then close Excel and reopen it.
2. Name the range of cells containing values you want to evaluate.
Select the range of cells. Then in the box that shows the cell address (i.e., E9), type in the name "datacolumn" without the double quotes (" "). Make sure that you repeat this step each time the size of your data range changes. If you have already named the range before, you need to first delete it by going to Insert->name->define and deleting "datacolumn".
3. Press Alt+F11 to open up the Visual Basic Editor
4. Create a module to copy the code into.
Insert->Module
5. Copy the following code into the box that appears. Make sure that if "Option Explicit" is in the box, that you copy the code BELOW "Option Explicit".
Sub ShowCellValues()
Dim cell As Range
For Each cell In Range("datacolumn")
If cell.Value < -25000 Or cell.Value > 25000 Then
cell.Font.ColorIndex = 2
End If
Next cell
End Sub
6. Click on the button that looks like the play button on a DVD remote control. It should be right above the box you copied the code into.