I assume you have your S/N (or subscription information). In that case, you can download from http://helpx.adobe.com/acrobat/kb/acrobat-downloads.html. You should deactivate the version on the old computer so you keep your activations current. Also, be sure to register your installation, particularly with the S/N for the future so you have access to info online (like your S/N). Also make a backup of the install file on a CD or backup drive for the future -- a download may not always be available, and even if it is it may be hard to find.
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