Cryptex creates a virtual hard drive, what it calls a "vault," on your hard drive or a USB thumb drive. The 30-day trial of Cryptex lets you create one vault; you can create more than one when you pay for the program, which costs between $45 (up to 6GB vaults) and $119 (up to 100GB vaults). Once you've created a vault through the software, you can work from it as if it were a folder -- you just have to start up Cryptex to access it.
When I created a 50MB Cryptex vault on my USB thumb drive, the vault appeared as a separate drive in the My Computer area. This may get confusing during everyday use. Opening and using files in the vault seemed just as quick as with a regular, unencrypted folder on my hard drive, so speed isn't an issue.
If you do want to lock just one folder on your drive, Cryptex will be too much program for you. You might look at Folder Lock for that purpose.