You don't have to deactivate it, you are allowed to have it installed on X number of systems at any one time (X being the number of licenses you have). So in cases like this is isn't the number of installs you have done, but the number of systems it is currently installed on and active getting updates, etcetera.
Provided you have saved your license.avastlic file, you can just use your license file again and it will give you the remainder of your license period.