If you have downloaded CCleaner and you're being told to pay to for any part of it, then sadly, you haven't downloaded CCleaner. You have probably been browser hijacked and sent somewhere else. This information would correlate to why your computer seems slow. I would guess that your system is infected with malware.
Go to
http://www.malwarebytes.org/mbam.php
Download the program, install it and update it, then do a "Full Scan". It should pick up any nasties and help you get rid of them. Then you can move on to download the real Ccleaner. Beware though, browser hijacks (which re-direct you from a legitimate website to a fake one) may well do this for any site you enter, making downloading legitimate anti-malware software quite difficult. Whatever you do, DO NOT pay for anything or enter any payment details whatsoever. Both CCleaner and Malwarebytes are functionally free although you can opt to pay for Malwarebytes real-time protection, but this is optional and not required for your purposes.