If you see this message
and you are using IIS 6.0+ then you should do the following:
1.Open IIS Manager
2.Expand your website's folder and right click the folder that can not be accessed
3.Right-click this folder and then click “Permissions”
4.Click “edit” and then “add”
5.Add 'NETWORK SERVICE' and grant it “Full control”
Now the problem should be gone.
Hope this helps someone.