(environment is WinXP under ActiveDirectory)

I have a folder that is shared read/write with a group. I want to be able to lock down individual files in that folder as read-only, but no matter what I try, the files inherit the permissions on the folder. Even if they are set as read-only, people (not the owner) can delete them.

Is there a way to get the read-only attribute to actually make the file read-only even if it is in a read/write directory?