Some people do not have their computer date set right and if it wasn't set right at the time, PI may be thinking your trial has expired, in which case you need to put in your serial code number if you have bought it.

It doesn't get rid of the memory that it has been on your computer, it just means you can't completely remove it being there in which case it thinks you have installed it before.

A person who specializes in cleaning your computer system would be the one I think who could really help you. My son's a computer guru and is about certified in everything under the sun, so I don't have to worry. Also some things like firewall and anti-virus may still be to blame, you can check your settings as to what it will allow you to do.