let me explain what 不提也罷 said

It is best to do in Excel's visual basic.
But 不提也罷's idea will work.
1) enter your data say in cells A1 to A40000
2) enter 1 in B1,
3) select B1 to B40000, Edit _> Fill -> Series -> step value set to 1
4) in cell C100 enter:
=IF((RIGHT(B100,2))="00","y","n")
5) in cell D100 enter:
=IF((C100="y",STDEV(A1:A100),"")
6) select C100 to D40000, press Crtl-D ti fill the formula.
7) move block A1:D40000 to where you want, i.e. enter headers, only when you finish all the steps above.
8) if you want to display only the stdev, do this:
select C100 to D40000, Data -> sort -> sort by Column C -> Decending