AstroGrep is a small tool which will search for PC for files containing specified keywords.
The program doesn't contain any indexing component, so searches are relatively slow. But by way of compensation, AstroGrep is also very small (around 300KB), portable, and has a lot of options. It can search individual folders, particular folder trees or entire drives, for instance; you can search all, or just particular file types; there's regular expression support, and you can search for files based on their modification date, file size and more.
Enter a few keywords, click Search, and AstroGrep will then go to work, scanning your specified folders for matches, and displaying various statistics (the file being searched at the moment, the number of matches so far, and so on) on its status bar.
And if a match appears, you don't have to wait for the search to finish before you can review it. Just click a document and AstroGrep will display the relevant area of the file, with the keyword(s) highlighted, and optionally a few lines from either side (the authors say this is great for viewing source code, but it's handy elsewhere, too).
If you just want simple, fast keyword searching then the standard Windows Search will probably be a more useful option. If you can use the portability and extra searching flexibility, though, AstroGrep is a great tool, being compact, powerful, yet also very easy to use.