Excel not updating

My excel formulas are not recalculating when I change the values of the range. I have no Idea how to fix this. I reinstalled sp1 (office2k) and it did not work. Heres the example of what is happening.
Cell Names A1 B1 C1
2 1 =Sum(A1:B1) or 3
New value 3 1 =Sum(A1:B1) 3 (should be 4) When I double click on it and hit enter it recalculates. HELP!!!!!