Sorry about the late answer, my host had some server issues today.
So, by default this is possible only on title, content and excerpt fields only. The reason is the speed. However I put together a modified version of the core search class to support this feature for terms and custom fields as well.
You can change the search logic on the General Options -> Behavior tab: https://i.imgur.com/Q9SGAGa.png
You should try out all of the variations, one of them should fit your needs.
As I mentioned before it won't affect the term and custom field search, unless you apply a modification. Open up the wp-content/plugins/ajax-search-pro/includes/search_content.class.php file and replace it's contents with this: http://pastebin.com/raw.php?i=GBAMFNYc
It should work based on my initial tests on my local test server.Best,
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