It's not your css, for the most part. It is mainly your layout. The search button is css, inline css. Simply removing the inline style attribute:
will fix that. The rest is layout, you have 8 instances of colspan="10". Several of these should be removed, others reduced. In order of their appearance on your page:
<input class="box" style="width:100%" type="submit" value="Search">
1st through 5th - remove.
6th - change to colspan="7" (this is the calendar)
7th - change to colspan="2"
8th - remove
Note: Since you have no "Latest 5 active topics" showing, if they use extra columns, the 4rth colspan="10" should be adjusted to the number of columns used there, rather than be removed.