Optimize Slow Conditional Formatting Code

I am having a little trouble with this code, which runs in my simple but efective ressource overview. It loops through a rather large range and assigns interior colour to the cells based on certain criteria. On my stationary machine (Excel 2003) it takes approximately 15-30 seconds to run the code which is acceptable. On my laptop (Excel 2007) it takes 5+ minutes which is unacceptable. Is there a workaround so as to optimize the speed? Further, when I run this workbook on Excel 2007, even entering an integer in the sheet takes 3-4 seconds, and no code is running! In my first version I used a Change_Event to colour cells on the fly but this was slow and prevented multible cell editing as well as pasting values into the appropriate range.

Public Sub Farvelade()
Dim icolor, Navn As Integer
Dim TargetRow, LastRow, Previous As Long
Dim Target As Range
MsgBox "Det kan tage 15-30 sekunder at opdatere ressourcekalenderen"
Range("A5").Activate
Range("A5"). CurrentRegion.Select
'Set range
ActiveCell.Offset(Selection.Rows.Count, 0).Activate
LastRow = Cells(Cells.Rows.Count, "A").End(xlUp).Row.....................