Hi Farzad,
The whole subject of PixInsight Licensing is covered in pretty good detail on the FAQ section link from the PixInsight HomePage.
Fundamentally, other than your account login details for the PixInsight download repositories and your login details for the PixInsight Forum, the most important piece of information that you want to look after is your PixInsight License Key. It is this file - alone - that allows you to freely install PI on any and all of as many machines as you own - obviously on a strictly-enforced 'non-commercial' basis.
Have a look, and get back to us if you are still struggling.