I bought the cd with adobe photoshop CS5 in the past, but now I got a netbook without a disk drive

I bought the cd with adobe photoshop CS5 in the past when I had a computer. But now I got a netbook without a disk drive. How can I install this program without purchasing an external disk drive? Somebody told me that I can download this version of the program from the website and then enter the key written on the disk in the application itself. But I can't find this opportunity on the site.