If you do not change your computer's components or reformat your hard drive, you should not need to Unlock Live again. If you are deleting Live, please save the Unlock.cfg file located in the following paths:

Windows (Live 3): (Installation Directory)/Ableton/Unlock
Windows (Live 4 and later versions): C:\Documents and Settings\your user name\Application Data\Ableton\Live\Unlock
Mac OS 9: /System/Application Support/Ableton/Live/Unlock
Mac OS X: /User/Library/Application Support/Ableton/Live/Unlock
Note that on some Windows systems the application data folder is hidden by default. Here is how to make it visible:

Windows XP/2000

Launch Explorer.
Select Tools/Folder Options/View.
Check "Show hidden files and folders."
You should not need to Unlock again if you have not changed your computer's components and you restore this file to the correct location after reinstalling Live.