If you registered your product, you should be able to find the download and serial number from your Adobe account.
You might still have to contact Customer Support to reset your Activations, unless you managed to deactivate the original installation.
Download cs5 install it you can sign in to Adobe.com then use the My Products and Services link to retrieve your serial. Most likely you will need to call Adobe Customer Support to Activate it for you most likely did not deactivate your old install.
Answers well given. Reinforcing those above, I can say from personal experience that Adobe Customer Support (voice-called on the telephone) is pretty good at resolving activation issues.