Apply formula as users do entry for each row. Excel 2007

Hi,

I am wondering if a vba is possible to accomplish this little task:

There are three columns, two of which require manual entry. The 3rd one is derived.

I would like to have a vba to apply the formula for each row when users are doing data entry instead of applying formula to the entire column right upfront, which tremendously increase CPU workload at the beginning.

formula is:

=IF(AND(A2="y",B2="y"),"Yes","No")

In addition, I would like to have the formula column(col C) somehow locked so that users don't accidentally override the formula with their "own" values. Is this possible??

I have attached a spreadsheet for reference to test it out.