I saw Mac Noland's post tonight about turning off the just-in-time debugging popup window on his TFS server. We've had this problem internally. If you have a debugger installed on your computer, such as Visual Studio or WinDbg, the installation will enable just-in-time debugging such that when something crashes you'll get a window asking you whether you want to debug the problem.
That's a great feature, except when there's no one around to dismiss the dialog. If you have a build computer, for example, and someone's custom msbuild task crashes in the middle of the night, you'll probably just want the msbuild process to exit rather than hang the build. It gets even worse if you connect to the computer via terminal services and don't connect to the console session, because you won't even see the dialog. The same is true on your application tier as well.
To turn off JIT debugging, you'll need to delete the two registry keys described in Visual Studio Debugger Just-In-Time Debugging.