absolutely a fresh install, everything was deleted on the httpd directory , a fresh empty database i really can't tell what's wrong with it. there isn't any other database on my dedicated server to cause this
Allright, it seems that is working now but this is how.
On my dedicated linux server I run Plesk 8.1 Control Panel. First time when I bought the server I've set up a single domain for a couple of months until a couple of days ago when I bought two more domains from GoDaddy. I've changed the name servers from my GD account to the ones used by my server , and I've also added the 2 new domains from Plesk CP to my server. It seems something went wrong here. Although my new domains respond to http/ftp/email/mysql requests it seems there's an error with vBulletin. So after I spend all this day trying to set up my new forum on my new domain (unsuccessfully) I've decided to install it on my first site which was on the server. I've created a new database and started the instalation on my first site (not the one that I was intenting to install it) and it worked. I have moved the entire /forum/ directory from /home/httpd/vhosts/my-first-site.com/httpdocs/forumm to /home/httpd/vhosts/the-site-that-ive-wanted-to-install-it.com/httpdocs/forum/ and that's all. It seems every function is working just fine now. I can save my settings from my admin control panel, i can create/delete/modify the forums, import avatars/smilies and so on.
I really don't have any ideea why it didn't worked for the first time on my second domain since I did the same steps. I hate Plesk. vBulletin is ok, it was Plesk's fault !
Now I will focus to set it up correctly and after to optimize it using your advices.
I guess I was wrong

it worked for a couple of minutes , it seems that I'm having some issues with mysql server
I got this message like 2000 times in my mysqld.log
Number of processes running now: 0
070115 11:41:20 mysqld restarted
070115 11:41:20 InnoDB: Started; log sequence number 0 2874641
070115 11:41:20 [ERROR] /usr/libexec/mysqld: Error writing file '/var/run/mysqld/mysqld.pid' (Errcode: 28)
070115 11:41:20 [ERROR] Can't start server: can't create PID file: No space left on device
I'm looking for a solution to fix it and my forum will be up after