What is this new user account for? What created it?

The aspnet_wp or ASP.NET Machine Account is created when the Microsoft .Net Framework 1.1 is installed onto a Windows XP computer. The user is created to run the asp.net worker process used in Microsoft's Internet Information Services, which allows ASP.net to run on your local web server (This is pretty much its only use, it is not used to run normal .net managed executables). There is not a need to worry about this user's presence; it was not created in malicious way.