If you use column A to set the last row, i.e. the last row of column A with data populated in it, you can use something like this ...
<font face=Tahoma New><SPAN style="color:#00007F">Option</SPAN> <SPAN style="color:#00007F">Explicit</SPAN>
<SPAN style="color:#00007F">Sub</SPAN> CheckColsAandB()
<SPAN style="color:#00007F">Dim</SPAN> lastRow <SPAN style="color:#00007F">As</SPAN> <SPAN style="color:#00007F">Long</SPAN>, i <SPAN style="color:#00007F">As</SPAN> <SPAN style="color:#00007F">Long</SPAN>
lastRow = Sheets("Sheet1").Range("A65536").End(xlUp).Row
<SPAN style="color:#00007F">For</SPAN> i = 1 <SPAN style="color:#00007F">To</SPAN> lastRow
<SPAN style="color:#00007F">If</SPAN> Sheets("Sheet1").Range("A" & i) = Sheets("Sheet1").Range("B" & i) <SPAN style="color:#00007F">Then</SPAN>
Sheets("Sheet1").Range("C" & i).ClearContents
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">If</SPAN>
<SPAN style="color:#00007F">Next</SPAN> i
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">Sub</SPAN>
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