Contact your host. browse the logs and make sure to secure all security risks. Even if they do not want to.
Remove all unneeded files and programs from your server.
Close all ports that do not need to be open and restrict ports that only you need to your IP. Only allow ftp from your IP address.
Follow these instructions as well:
http://www.vbulletin.com/forum/showthread.php?t=172234
For everything that you install, check
http://www.securityfocus.com/vulnerabilities and also search google for the name of the software and the words 'exploit OR vulnerability'