Mid, Left, Right Function Macro Error

I've got a little application in excel using macros.

It worked perfectly with Excel 2000, but with Excel 2003 shows an error for every formula I've used in the macro -right, left, mid, date...

Sub buscar_libros_datos()

On Error Resume Next

Dim libro As Variant
Dim ruta As Variant
Dim libroruta As Variant
Dim tabcol As Variant
Dim tabfil As Variant
Dim posguion As Variant
Dim posparen1 As Variant
Dim posparen2 As Variant
Dim categoria As Variant
Dim categant As Variant
Dim tipo_info As Variant
Dim fecha_carga As Variant
Dim i As Variant