There are many ways why this fails.
Which OS are u using?
I am talking about XP or above
1. After adding to security tab, there is an advanced option. Click that.
Select the user account you require (ASPNET here). select "Allow
inheritable ..." check box and apply it.
2. You should be using IIS with Windows Auth.
3. Check your task manager for which credential the Worker process
(aspnet_wp.exe) or (w3wp.exe) is running. Give that user the rights to that
folder.
If you are using Windows Server 2003 then your app pool determines the
credential for worker process.
Hope this helps.