Agent Ransack (free) allows for creating customized searches that go well beyond basic Windows searches. Its most powerful features are aimed at programmers and other techies. It's available in a 32-bit and a 64-bit version.
Basic searches are relatively easy. You choose all or part of a file name to search for, optional text within a file, and the drive or directory to search. Agent Ransack can dig into binary files, including .exe files, to pull out matching text, but can't search within .zip files or other archives (for that you'll need the paid version of the software, FileLocator Pro). Matching file names display on the lower left, and clicking a listed file displays any matching content on the lower right without having to open the file, a nice plus. Double-clicking a filename opens the file as per normal.
Search savants can go a great deal further with some of Agent Ransack's more expert features, including an option to use Perl-style regular expressions instead of simple DOS expressions. You're likely familiar with DOS expressions, which use '*' as a wildcard, for example. On the other hand, regular expressions allow for creating much more extensive and precise searches, but they're about as intuitive as the U.S. tax code.
For example, the regular expression "^(July|August).*Documents?.*[0-9]+.*\.doctiny_mce_markerquot; will find a file that begins with 'July' or 'August' followed at some point by 'Document' or 'Documents' followed at some point by a number and has a '.doc' extension (per the program's help).
Agent Ransack does include some nice ease-of-use features, including a wizard that can be a big help constructing regular expressions. Its well-written help file does a good job at explaining the program's features, and the app can save your carefully prepared search criteria or export its results. Techies who are already proficient with regular expressions may find it especially useful, but novice users conducting basic searches will also benefit from being able to quickly sift through matching content without having to open files.
The free Agent Ransack is a slimmed-down version of the US$40 FileLocator Pro, which can search in archives and Outlook .pst files and perform even more customized searches. Programmers and others who need to find one specific bit of text among a huge slew of archive and other files may need FileLocator Pro's extra features, but average Joes will probably be content with Agent Ransack.