Corrupt data question..
 

Not every time, but very often after i save a file i have been working on the
data corrupts somehow..

Even though the input data for the formula is right, the formula says it has
an error, and i have to delete lines till i find the one causing the problem
then re-enter the data.

This only happens after i have saved it and and reopen it the next morning.
Really annoying as i have to go and fix it nearly every morning, which takes
about 10-15 mins, depending on the amount of data i have to sift through..

I am using Excel 2003, and it is a =SUMPRODUCT(--(X3:X299+AJ3:AJ299=2))
formula. Basically putting 1's into different cells to make different stats.
there are quite a few of these formulas just corresponding to different cells
(people/sales). And even though there is only a 1 in the cell, the formula
says it has an error, yet all the info is right..

Any idea's??