Compared to phpBB3, links in phpBB2 had a slightly different behaviour: When you clicked them, they opened a new window with the requested page. This was changed in phpBB3 for a couple of reasons, but one of the most important ones was the goal to achieve XHTML 1.0 Strict-validation for the whole board.
But...
in the file 
includes/functions_content.php change 
Code:
    $html    = "$whitespace<!-- $tag --><a$class href=\"$url\">$text</a><!-- $tag -->$append";
 to 
Code:
        if ($type == MAGIC_URL_EMAIL)
        {
            $html    = "$whitespace<!-- $tag --><a$class href=\"$url\">$text</a><!-- $tag -->$append";    
        }
        else 
        {
            $html    = "$whitespace<!-- $tag --><a$class href=\"$url\" onclick=\"window.open(this.href);return false;\">$text</a><!-- $tag -->$append";
        } 
 And in 
styles/610nm/template/bbcode.html you have to change 
Code:
<!-- BEGIN url --><a href="{URL}" class="postlink">{DESCRIPTION}</a><!-- END url -->
 with 
Code:
<!-- BEGIN url --><a href="{URL}" onclick="window.open(this.href);return false;" class="postlink">{DESCRIPTION}</a><!-- END url -->
 And then finally, in 
a little fun file called 
includes/functions.php about 2700 lines of code in, you gotta change 
Code:
      case 'bbcode_htm':
         return array(
            '#<!\-\- e \-\-><a href="mailto:(.*?)">.*?</a><!\-\- e \-\->#',
            '#<!\-\- l \-\-><a (?:class="[\w-]+" )?href="(.*?)(?:(&|\?)sid=[0-9a-f]{32})?">.*?</a><!\-\- l \-\->#',
            '#<!\-\- ([mw]) \-\-><a (?:class="[\w-]+" )?href="(.*?)">.*?</a><!\-\- \1 \-\->#',
            '#<!\-\- s(.*?) \-\-><img src="\{SMILIES_PATH\}\/.*? \/><!\-\- s\1 \-\->#',
            '#<!\-\- .*? \-\->#s',
            '#<.*?>#s',
         );
 with 
Code:
      case 'bbcode_htm':
         return array(
            '#<!\-\- e \-\-><a href="mailto:(.*?)">.*?</a><!\-\- e \-\->#',
            '#<!\-\- l \-\-><a (?:class="[\w-]+" )?href="(.*?)(?:(&|\?)sid=[0-9a-f]{32})?" onclick="window\.open\(this\.href\);return false;">.*?</a><!\-\- l \-\->#',
            '#<!\-\- ([mw]) \-\-><a (?:class="[\w-]+" )?href="(.*?)" onclick="window\.open\(this\.href\);return false;">.*?</a><!\-\- \1 \-\->#',
            '#<!\-\- s(.*?) \-\-><img src="\{SMILIES_PATH\}\/.*? \/><!\-\- s\1 \-\->#',
            '#<!\-\- .*? \-\->#s',
            '#<.*?>#s',
         );