Is there a .Net API for Windows Error Reporting
It's not likely. WER isn't meant to be used by applications, its something that a user can choose to enable, and when (s)he does, all application crashes are turned into Windows Error Reports and sent to a central server. It's not something you turn on and off for individual applications. You generate an error report by crashing on a machine where the user has enabled WER.
Developers just have to register so that they can get the crash information on their products.
from http://msdn.microsoft.com/en-us/library/bb513641(VS.85).aspx
Developers can register with Windows Quality Online Services to get information about the problems customers are experiencing with their applications and help customers fix these problems. Developers can also use Application Recovery and Restart to ensure that customers do not lose data when their application crashes and allow users to quickly return to their tasks.
I'm sure you could use interop to call the WER api's, if you want to generate a report without crashing.
The WER team have a project on Codeplex; there's a managed Services DLL downloadable from http://www.codeplex.com/wer/Release/ProjectReleases.aspx?ReleaseId=12825