Excel 2007 : How to re-initialized column

Dear All,

Please let me know how to do that.
Here is the example
Below Serial number column should come automatically and re-initialized when column A (Name) get changed.
--
Name Pay rate Serial no
John 12 1
John 13 2
John 14 3
Mike 10 1
Mike 12 2
Mike 14 3
Mike 16 4
Sam 10 1
Sam 11 2
--

Thanks
Sam

If your Names start in A2 and you want the Serial Numbers in column C, you can use the following formula in C2 and copied down

=IF(A2=A1,C1+1,1)

Thanks buddy, it worked !