Avoid using Time Machine for re-installing Adobe software. It rarely works and usually breaks the activation mechanism.
I've been told you can only use the CD to install CS5 onto your computer twice,
Not quite.
You can install the software as many times as you like but you can only activate the software for use on a maximum of two computers at the same time.
To re-install you will need your CS5 serial number.
Download the installer from Download CS5 products
Install then enter your serial number when prompted.