Error with restored database

I tried to make the update to 2.0.6, but I wasn’t able to complete it (I will try again later), so I deleted all files in the directory and restored all original files and database.

Now, after first visit, slimstat is no more able to count them, and I’m able to post only one comment; with the second I get:

WordPress database error: [Duplicate entry ‘0’ for key 1]

….

Warning: Cannot modify header information – headers already sent by (output started at
…./wp-includes/wp-db.php:102)
in …./wp-includes/pluggable-functions.php on line 290

For what I understood, first comment get ID 0 instead of “343” and second one gets again 0 as ID, and there will be this problem with articles too I suppose.

Following instructions, I didn’t deactivate plugins before backing up, so I find them active when I first access the admin panel.
I used the hosting function to backup and then restore database. What can be the error?

Thanks a lot