how to avoid trail of software

i am using a software for trail version of 7 days, after that if i install it says license is invalid. So i formatted my complete hard drive and installed os windows 8.1. Later i installed software again, actually i need to get a trail version of 7 days, but it is saying that license is invalid.

I want to know weather after even formatting is some registry files are not cleared..??

please help me out to use that software