How to take standard deviation of only positive cells

Hello all...this is my first post.

I have excel 2007 and am trying to find the function for calculating the standard deviation for only the positive numbers in a table of data...

Any help on this is much appreciated.

Something I learned last week from a forum guru...
Suppose A1:A6 is the data you want, but only including the positives. Put an IF() in your function like so,
=STDEV(IF(A1:A6>0,A1:A6))
and make sure you hit Ctrl-Shift-Enter to turn it into an array formula. {Curly brackets appear round the formula}
As if by magic....

awesome...thanks for your help....the ctrl+shft+entr was a big help!