You can have it deactivated by contacting Adobe Support thru chat and asking them to reset your activations (only ask for that or they might send you back here where no one can help).
Chat support - For the link below click the Still Need Help? option in the blue area at the bottom and choose the chat option...
Make sure you are logged in to the Adobe site, have cookies enabled, clear your cookie cache. If it fails to connect try using a different browser.
Serial number and activation chat support (non-CC)
https://helpx.adobe.com/contact.html?step=PHSP-PHXS_downloading-installing-setting-up_licensing-activation_stillNeedHelp
Note: Your license allows for having two machines with activated installations, so if you only have the one and run into a problem getting Support to help then you can activate the second still. If you pursue the reset do it before activating the second to save having to reactivate it.
Downloads:
CS5 - http://helpx.adobe.com/creative-suite/kb/cs5-product-downloads.html
You can also download the trial version of the software thru the page linked below and then use your current serial number to activate it. Be sure to follow the steps outlined in the Note: Very Important Instructions section on the download pages at this site and have cookies enabled in your browser or else the download will not work properly.
CS5: http://prodesigntools.com/all-adobe-cs5-direct-download-links.html