they dont do anything to your pc but the risk is when you download using these apps that the files will contain viruses but as long as you run anti-virus you will be fine.
Limewire doesn't ruin you computer. although it does have many virus but if you have a good virus scanner it will pick it up and corrupt it...So altogether it's good.
Great website and it's easy to use. Put a limitless supply of media/movies in your control and on your phone! Great stuff. I HIGHLY recommend this subscription. http://www.kazaa-platinum.com