It's a matter of context.
A summary field placed on a layout based on the same table in which it is defined computes a total based on the current found set. Thus, you can get your total if you perform a find that excludes the negative values.
Or you can refer to this same summary field from a related table and if the relationship excludes the negatives, you'll get your table.
And a relationship that matches to all records can produce this total if you put the summary field in a one row filtered portal where the portal filter excludes the negatives.
Or you can set up a calculation field such as: IF ( Field #2 > 0 ; Field #2 ) and then you can use a summary field to compute the total of this calculation field.