Tips: You cannot add a password to an existing zip file. 7-Zip allows you to set password while creating a ZIP archive. You will need to create a new zip file to password protect it.
Step 1: Make sure 7-Zip is installed on your computer.
Tips: If you have not installed it, 7-Zip is available from the Software Center (Start > All Programs > Microsoft System Center > Software Center). Make sure you have 7-Zip installed before continuing.
Step 2: Right click on the file/folder you want to compress into a password protected zip file, and then from the context menu point to 7-Zip, and select Add to archive.