You can only use a single license on one computer. What if you now have a new computer but the license key was applied on the old one? The easy solution is transferring the license to your new computer and you can do this yourself as long as you still have the license key.
To begin, run Tagitize on the new computer. When the window below pops up, click Enter License Information. In the next window, follow the instruction in the Transfer section to finish migrating your license.
Doing this will make the license key stop working on your old computer unless you perform the license transfer in reverse.