I boot my PC with an another hard disk which is containing Fedora 13.

After that I could not boot my PC with my Ubuntu 10.04 hard disk and I have got the error message as
"/usr/lib/libgconf2-4/gconf-sanity-check-2 exited with error status 256".

And I could not login to my current user account .

However it was possible to log as root.

So I logged as root
 (username : root)

And I noticed there is an another new file in my file system (on root "/") with the name "core"
so I renamed It to "backup_core".

Then Restarted the mechine..

Then Everything was OK. and I could log to my current user account with out any trouble