Empty Trash Problem

Can any one help me ??
I had a problem that when I empty the trash , the trash is not empty....even when using the option key.
I found later that the trash folder has been renamed ( ?.?Trash ), I renamed it back to ( .Trash ) which solved the problem , but.... when I restart the computer it renames the trash folder back to ( ?.?Trash ), I tried to recreate the user which worked for two days only,, and the same problem appeared again.

If any one had this problem, could he/her solve it without reinstalling the system?????

thanks