Setting up a google search is really easy. And it will search your entire site, or any other sites that you want it to.
To start with go to google's custom search page http://www.google.com/cse/
Then just set up how you want the search to work. Some of the options include loading the results in a new page, or using ajax to update the current page. Figuring out what settings you prefer will be the most time consuming thing about this.
Then whenever you are done just create an island and copy the supplied code into it. Then add the island to your portal layout. You can then turn off the forum search if you want.