You will have to remove the software from the old computer, and simply reinstall onto the new one.
Have you still got the retail version of the software you downloaded? The download llink should be in your 'welcome' email you received when you purchased the software.
You will also need your software key (or install password). Again, you will find this in your welcome email.
If you haven't got these details, then we can send you these details.
http://www.blumentals.net/support/lostkey.php