Using "for loops" in functions

[KnAsTa]

hi i have a function:

=IF(ISNUMBER(MATCH(A5,week1.xls!$A$1:$A$10,0)), IF((VLOOKUP(A5,week1.xls!$A$1:$D$10,3,0)>VLOOKUP(A5,week1.xls!$A$1:$D$10,4,0)),3,0))

and i need it to cycle from week1.xls to week19.xls. (19 xls files all with the same ranges)
When i cycle through them, i also have to add up all the returns to get a total return. IE, if a team wins (VLOOKUP > VLOOKUP section - ie score in column 3 greater than score in column 4. column 1 and 2 are two team names.)

Any help would be much appreciated.