Document folder keps open on its own at startup

The answer is a corrupted registry entry.

Run regedit and navigate to:

HKLM/SOFTWARE/Microsoft/Windows NT/CurrentVersion/Winlogon

In there there should be a value (on right hand side of screen) called Userinit.

The data for this value is probably something similar to:

C:\windows\system32\userinit.exe,C:\windows\system32\userinit.exe,

If you do see a duplicated string in there similar to the above - simply double click on the Userinit value and edit the data so as to delete everything to the right of the first comma (,). In the case above you would leave only:

C:\windows\system32\userinit.exe,

Exit registry edit and reboot. No more My Documents!