can't get formulas to fill down. help!!

I recently upgraded to office 2003 and suddenly cannot fill a formula down a column like i used to be able to. any idea why? for example, i have a column A with values 1:5 and column B with values 1:5. in C1 I type =A1+B1 and hit enter. Then, I try to fill this down the rest of the column. However, instead of returning 2,4,6,8,10, i get 2's all the way down the column. Whats even more weird is that the C2 has the formula =A2+B2 and C3 has =A3+C3, but the values are still 2. Whats going on?!? Instead of running the formula, its just copying the cells. If i fill the correct formula in C1:C3 then fill down the remaining couple rows, it'll just copy the above 3 cells, so i end up with 2,4,6,2,4. I just don't get it.

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