Go to msconfig and take a look through the startup tab and see if you can see an exectuable that might be it. Then you can untick it.
Go to task manager and look through the list of processes and see if you can see which one it is.
I took a quick look around to see if there was a utility that can identify the exectuable that owns a window. Spy++ might do it, from Microsoft. The other one I thought of is strokeit - which is actually a mouse gestures utility, but it has a facility for identifying a window.
Once you identify it, you can disable it from msconfig.