folder permssion

I have  a shared folder in windows 2008 standard and i want to give access to Read and write permission but not to modify permission ,to the data in the same folder.

I have given the read and write permission & no modify permission.so Read,write & Modify permission are getting applied to the new folder and the data.

But for the existing data Read Permission & No modify setting is getting applied .But Write permission is not getting applied to the existing data .

Hw ever i created the creator/owner permission to get the data modified .It is happening.