function to search and replace

Help! I am stuck writing a function that will search a column for specific text and replace it. Basically search column B for the text in cell C2, if the text in cell C2 is found in column B, replace the cell in B with the cell in D that is next to the cell in column C that was used for the search. I need this function to to go down column C and run the same process for all the cells. It sounds kind of confusing, but the attached xls should help explain it.