You can eitehr create a formula or use a Crystal Running Total.
If a Running Total, in the evaluate use a formula palce:
{table.field} = "Yes"
If a formula, use:
if {table.field} = "Yes" then
1
else
0
Then place the field in the details, right click it and select insert->summary->sum
Now you can delete the field from thye details.
If you want to exclude all of the NOs from the entire report, go to Report->Selection Formula->Record and place something like:
{table.field} = "Yes"
-k