And if you live in the US then downloading copyrighted material that you have not paid for or have a right to it by previously paying for it is not legal. Buying Pro is not paying for the right to copyrighted material. It is simply paying for the pro version of LimeWire.
Hopefully you have not gone to one of the many scam sites and purchased LimeWire. LimeWire basic is free to use for as long as you wish. And Pro cost $18.88 for Pro and 6 months of free upgrades. You can get either of them at the link below.
http://www.limewire.com/english/content/download.shtml