i want to install my cs5 on an another pc

You can install and activate (purchased) perpetual software two computers of  the  same type (mac or pc), as long as you only use  it on one at a time. (lightroom is the exception as it is a multi platform product, so it can be installed on pc and mac platforms)

It is a good practice to deactivate software on a computer where it will no longer be used, especially before giving away or selling the computer, or reformatting the hard disk. Please see this link:  how to deactivate software.  This frees up  one activation so  you have the option to install the software on another computer at any time.

After downloading from here:  Download CS5 products

Please see these instructions to extract the files and install your product.

Alternately, you can download from here:

http://prodesigntools.com/all-adobe-cs5-direct-download-links.html

Pattie