If you are using FileMaker 11, check out quick find. You can have the user enter data into a global field and then your script can use it in quick find to search all the fields on your layout.
This script could be expanded to search multiple layouts, but that kind of search usually isn't as useful as it first looks. I've fielded that type of question before. The issue comes down to this in most cases: If you are searching more than one table, how do you intend to display the results in FileMaker?