Hi ,
If the column which has your data is column A , starting from cell A2 , then in any unused column put in the following formula :
=IF(A2>=100,100,A2)
or
=MAX(100,A2)
Copy this down. The unused column will now have 100 where ever the data in the corresponding cell in column A is greater than or equal to 100 , else it will have the value itself.
Narayan