You can search for files in File Explorer using the search box next to the address bar in the upper right. (Note that My Computer was renamed to This PC in File Explorer in Windows 8.1 and Windows RT 8.1.)
Open File Explorer by swiping in from the right edge of the screen, tapping Search (or if you're using a mouse, pointing to the upper-right corner of the screen, moving the mouse pointer down, and then clicking Search), entering File Explorer in the search box, and then tapping or clicking File Explorer.
Double-tap or double-click the folder, drive, device, or network location you'd like to search.
In the upper right, next to the address bar, enter search terms in the search box.
Your results will start to appear with your search terms highlighted.
If you want to search for all files on your PC, open your operating system drive (typically called drive C), and then enter search terms.
You can also search your PC using the Search charm. For more info, see How to search.
If you want to search for all files with a particular file extension, such as .txt, enter your search term like this: *.txt.
When you tap or click inside the search box, the Search Tools tab appears. Here are some things you can do in this tab:
You can search for a file based on properties such as the date it was last modified, its size, or what kind of file it is (such as "Picture"). After you enter your search term, tap or click the Search Tools tab and use the options in the Refine group, and then enter your search terms in the search box. For example, if you want to search for files you modified this week, tap or click Date modified, and then tap or click This week.
You can narrow search results using other properties. For example, if you want to search only file names and not file contents, tap or click Other properties, choose Name, and then enter your search term.
If you can't find what you're looking for in a specific library or folder, you can expand the search to include different locations. When the search results appear, tap or click the Search Tools tab, and then choose an option under Search again in. (Some options might not be available in Windows RT.)
If search results don't contain matches from file contents, the location you're searching isn't in the search index. To include file contents for a single search, tap or click the Search Tools tab, tap or click Advanced options, and then choose File contents. To add the location to the index so that file contents are always included in searches, tap or click the Search Tools tab, tap or click Advanced options, and then choose Change indexed locations.