What is the most important thing in your computer or pc? It’s your data. Sometimes it is important to protect your important data from theft or leaks. Some data is confidential and you don't want others to even view it without authorization. This information may be your personal or companies data, bank or credit card information.
If you look on Internet there are many folder lock software available. May some are free but mostly are trial version and have to purchase them.
But after searching very hard I found one useful software which is TrueCrypt.
TrueCrypt is a free open-source disk encryption software. With the help of this you can create virtual encrypted disk within a folder and mount it as a real disk. You can encrypt an entire hard disk partition or some portion. In this software you can define the size of virtual disk and set the password. To access the data you have to mount this disk with the correct password and no data stored on an encrypted volume can be read without using the correct password/keyfile.
Note: Recently TrueCrypt displayes warning that “Using TrueCrypt is not secure as it may contain unfixed security issues”. So you get no longer supports for TrueCrypt. Still you can continue using it as normal, but do not download any new version.