To install the last Darklord rooted ROM you have to wipe everything.
To restore all the App (Viber conversation) and settings (Wifi pwd, email account, contacts. SMS...) is sufficient the Backup done with PC Companion or I have to do a Backup with a specific program?
Thanks
First of all, if you have a Google account, your Wi-Fi, contacts, and some other settings are already backed up to the cloud and will be restored automatically. SMS and app settings are different.
PC Companion backups are weird. I tried them a while ago - couldn't figure out how to restore. If you're so worried about particular apps, do a dirty install (wipe only /cache and Dalvik cache), then backup your SMS and all apps that you'd like using the new version of Sony's Backup & Restore app in 4.3. Then reboot back into recovery, wipe /data, /cache, and Dalvik cache, then install the ROM, then boot and do the basic setup. Once Android is finished restoring everything it could automatically, restore the backup from the Backup & Restore app. Just note that if you receive any SMSes before completing the restore, you'll lose them. Thus, it's best to put the phone into Airplane Mode and connect to Wi-Fi in order to complete the restoration.