This can happen when you left the home partition and used the same user. Most of the /home/ju settings are from a different operating system, probably KDE desktop also. Now, even if you re-install Mandriva and try to use the same user, it will not be accessible because Debian has wrote a few of it's own files in there, and some are probably Gnome related if you did not do KDE in Debian.
If this is what you did, your only option is to create a new user by a different name (jo for example) with command as root from failsafe terminal or single user mode:
You will be prompted for password and maybe a couple little things to which you use the default. Then, as root, transfer all data you want to keep from /home/ju to /home/jo and log into jo's account. You can then delete ju's account and recreate it and transfer all data back to the new /home/ju as root and delete /home/jo and log into your new ju account.