PNRP is a distributed name resolution protocol allowing Internet hosts to publish “peer names” and the corresponding IPv6 address. This basically allows every machine running PNRP to have it’s own name on the internet without having to register a specific domain name. Originally available in Windows XP SP2, it is now part of Windows Vista, and requires IPv6 to work. Here’s an article that talks about PNRP, and even describes how to get it working. There is also tons of information on Microsoft’s Technet. As most users aren’t able to use IPv6 yet, the service probably isn’t necessary for you, but since it’s set to manual by default, there’s little reason to disable it.