<img src="http://dl.dropbox.com/u/36472942/construct/sig/html/images/search1.png" border="0">
Using the top bar "Search" function allow you to come to that screen, affining your search accross the forums themselves (as opposed to the top search that searches through the website/tutorials/blog articles, leaving the forum aside).
[EDIT] With the new design, the global search is the top right "Search forums" text area, and you can access the advanced search form displayed above by clicking the "Advanced search" link right under.
Make sure to search on both functions when looking for informations !