There is no limit to the number of times you can download the software. The limit is on how many installations of it you can have activated at a given time.
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