jaredtheactor,
Welcome to the Apple Discussions!
This is an unusual problem, without any similar reports of which I'm aware. "loginwindow" is the process that handles user logon during startup, starts the Finder and Dock, and continues to run to handle Force Quit events, logouts, system error messages, and other system events. If something has corrupted the loginwindow code, causing the process itself to appear in the Dock, that would as you noted interfere with the Finder in all user accounts on that Mac.
There are two loginwindow preferences files in the system-level /Library/Preferences folder (and another two in each user account's /Users/username/Library/Preferences folder), com.apple.loginwindow.plist and loginwindow.plist . I have no idea if deleting the two system-level .plist files while in single-user mode will help; deleting a preferences .plist will (at least in theory) force the associated program to generate a new set of default preferences, but since there's no login preference for showing that hidden system-level process in the Dock, deleting the .plists is purely a shot in the dark. The same applies to the /Users/username/Library/Preferences/com.apple.dock.plist file which holds the user's Dock settings; since you've already tried using other user accounts; it's unlikely that all your user accounts just happened to pick up the same corrupted .plist at the same time. It's possible something damaged the /Library/Preferences/com.apple.dockfixup.plist file, which holds the default dock settings that are the baseline modified for each user account's Dock; I have no references as to what happens if you try to delete that .plist while in single-user mode, although it should also force creation of a new default Dock set.
In your place, I'd try deleting or moving to the Desktop the /Library/Preferences/com.apple.dockfixup.plist file while booted in single-user mode (e.g.,
rm /Library/Preferences/com.apple.dockfixup.plist followed by
mac-boot).
If that doesn't work, if you have another Mac running the same OS X version, you could try using
FireWire target disk mode and move /System/Library/CoreServices/loginwindow.app from the host (good) Mac to the target (bad) Mac. Since that's a complete WAG, you might be safer just going directly to doing an
Archive and Install (using the 'preserve users and settings' option).
You may want to post this problem to the
Using Mac OS X (10.3.9 and earlier) forum to see if anyone has better (preferably first-hand) advice.
Good luck!
