Are you using Crystal's Cross-Tab component or are you manually creating a cross-tab?
If you're using Crystal's cross-tab, create a formula like this:
If {DataTable.TotalSum} > 0 then {DataTable.TotalSum} else 0
Place the formual in your cross-tab and configure it for a sum.
If you're manually creating your cross-tab, use the same formula then add a summary field that will show the sum by group.
-Dell