WordPress Hacked

Hi,

One of my websites had an old version of wordpress (version 2.0.11 or below) and the blog got hacked.
Spammers were able to modify the php code in one of the footer files in the themes directory. They were also able to edit legitim posts and add hidden spam for search engines.

I would like to know if there are any vulnerabilities in wordpress 2.0.11?
And also how the hackers were able to replace / modify php code in the templetes directory.

Also I would like to know if upgrading to the latest version of wordpress will solve this issue, or if there are any security measures that I can take to prevent this from happening again.

Thanks in advance.