If you are using P2P and you have shared some folders your PC would run slowly. Its actually because the P2P software generates the hash links for all the files shared, which is then uploaded and the other users of the world can search from and download from you. And this process is done quite frequently. The hash links are updated many times.
For example, I used Ares as a P2P software on normal when the hashing functions run on the background you would see a CPU usage of around 30K - 55K. Whereas when the Ares was just downloading and not generating the hash links the CPU usage would be around 3K-5K.
So if you want to use P2P softwares don't share any folders. But that would be a bit unfair on the P2P world as it would only take and no give!!!
Well this is just a P2P perspective, never heard of torrents doing harm!! This is the first!!!