Finding files by only knowing extension?

• Open up Windows Explorer.
• Go to C:\
• Type in *.rar into the search box and hit Enter.

Then wait for the search process to find all the rar-files on your C-drive.

EDIT:

* is a wildcard, so you can also do more advanced things with it. Suppose you know your file is called <something>final.<extension> you can search for *final.* to find all files, whose names (excluding extension) ends in final or you can type kind: or size: into the search field to search by kind of file or size of file.