I've found that if you remove the very first line of the source code the underlining disappears.

HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

The main site doesn't contain this line so it displays OK, it's just the forum.