My Windows 7 installation has begun opening a blank debug.log in Notepad every time it boots, but I do not know why or what process is causing this. How can I find out?
From your comment I guess that this file already exists in the StartUp Folder. What happens is that Windows automatically tries to start files located here, in this case it simply uses the default program to open the file – in this case Notepad – which then pops up.
If this is the case, removing the file should suffice. You should also look which file might have created this, do you have some executable directly in that startup folder that might have created it? If that's the case you should move it to another place and simply add a shortcut to it to the StartUp Folder.