Adobe Acrobat Pro not working

I have installed adobe acrobat on a new computer.  I had to take the computer back as it had problems.  I was then given a new computer with the old hard drive installed.  I kept getting messages on my computer saying I need to activate a trial system.  The computer tech said I would have to reinstall acrobat.  Can someone please tell me how I can fix this problem? 

Thank you

Lorraine