One Macro Stops Another From Working

I have this macro which sits in the workbook module:

Code:
Private Sub Workbook_SheetSelectionChange(ByVal Sh As Object, ByVal Target As Excel.Range)
Static OldRng As Range

On Error Resume Next
If Application.CutCopyMode = xlCopy Or Application.CutCopyMode = xlCut Then
OldRng.EntireColumn.FormatConditions(1).Delete
OldRng.EntireRow.FormatConditions(1).Delete

[Code] .......

It's used to highlight the selected/active row.

But, as soon as I run another macro, it simply stops working entirely and I'm left with a row permanently highlighted until you manually remove the conditional formatting. All formats and cell colours controlled in the other macro stop working as well. This is the other (edited for publishing) macro, which is used to send an email, and is found in a module:

Code:
Sub Send_Mail()
'this sends an email that sends a text message

Dim OutApp As Object
Dim OutMail As Object
Dim MyButton As String
Dim strTo As String

[Code] .......

I didn't write the first macro so I'm not sure where the problem is? I'm guessing there is something in the code that is stopping it from working (i.e. running conditional formats) as soon as another macro is run? Is that was is happening?