Excell 2002

I am using numerous VLOOKUP functions (including the "FALSE" option) accross
a row to extract exact values from the same range, which may or may not
contain the look-up value. I then want to sum accross the row to give me a
running total. The problem is that VLOOKUP returns "#N/A" when an exact
match is not found; which then returns a "#N/A" error in the sum function.