Can you disable Windows Explorer from starting with Windows?

If you're running Windows XP/Vista/7:

1. Open the start menu.
2. Type "regedit" (without the ") and hit enter (UAC might come up).
3. Navigate to HKEY_LOCAL_MACHINE → Software → Microsoft → Windows NT → CurrentVersion → Winlogon.
4. Change the value of "Shell" to the .exe you want to use.

Reference: http://windows7themes.net/how-to-replace-explorer-exe-in-windows-7.html

For Windows 9x:

1. Open C:\windows\system.ini with the editor.
2. Replace the Shell=explorer.exe line with Shell=your.exe.

Reference: http://www.novell.com/coolsolutions/trench/3458.html

You can set taskmgr.exe to be your shell.

It is possible, yes. Though as you state, not ideal. Without the main explorer.exe running, you will not have a desktop/window manager unless you start or are running another one.

However, for Windows XP the steps to disable explorer on startup should be:

Go to Start -> Run -> Type in regedit and press enter.

Navigate to the following tree structure: HKEY_LOCAL_MACHINE -> Software -> Microsoft -> Windows NT -> CurrentVersion -> Winlogon

Change the value of the Shell key to blank, or another program to start instead of explorer.exe