Or we can find out what process the white box belongs to.
One program that can help is Microsoft Spy++... MS took the download link down a while ago but I still have it:
hereJust unzip it somewhere, and run spyxx.exe; (or spyxx_amd64 if you are running a 64-bit version, but the 32-bit version works in either one anyway).
If you get an error about the application configuration being incorrect, you'll need the Visual C++ 2008 redistributable:
http://www.microsoft.com/downloads/details.aspx?FamilyID=9b2da534-3e03-4391-8a4d-074b9f2bc1bf&displaylang=enOnce you've managed to start it, and you have the white box, you'll need to do the following:
1. select from the menu "Spy->Find Window"
drag the "finder tool" icon over the white box, and release.
Click "OK" in the finder tool dialog.
you should get a "Window properties" dialog. Ignore the gibberish and select the "process" tab, and click the underlined "process ID" value.
The "Module name" item here will reflect the base filename (usually) of the program that is creating the mystery white box; this can help guide you as to which startup items to select and not select.
Then again, it might be faster to just go with Allan's advice and just work through each one progressively; I just thought I'd throw in an alternative.