Konnichiwa,

Best way is to use the same utility you used to encrypt it. You'll have to remember the password.

You can use Archive programs to encrypt something, a pro is that it will also consume less space. WinRar has a better encryption than WinZip. Be sure to use a strong password as there are password crackers for such archives. If you use over 10 characters, mix upper/lower case, digits and special signs, avoid dictionary words (any language), names of favourite pet and the like, then it will take many lifetimes with current hardware to crack such archives.
The tools will crack a password of some 5 characters in less than a few hours, sometimes within seconds. Each extra character adds *255 possibilities when the whole ASCI set's used.

Just open the archive like normal and it will ask for the password.