How to stop disk conversion at next reboot

Clarifications

Since it seems that there are no prompts at all after rebooting where you could stop this, the only way you can be sure it won't happen is to boot from a live OS on CD/DVD or Flash drive that has registry editing capabilities. Possibly even a Window install/recovery disk that gives you access to the recovery console.

Or, if you slave the drive into another computer, and you can then load the registry hives into an offline registry editor.

I'm not entirely sure where the command would be found, possibly under one of the Run keys, Boot Execute, Scheduled Tasks, Winlogon, or Logon type entries.

For example:

HKLM\System\CurrentControlSet\Control\Session Manager\BootExecute

HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run

HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\RunOnce

HKCU\Software\Microsoft\Windows\CurrentVersion\Run

HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\Taskman

HKLM\System\CurrentControlSet\Control\Session Manager\BootExecute

remove the entry

autoconv \DosDevices\C: /FS:NTFS

and you probably want to replace it with the normal one

autocheck autochk *

If your system has already been shut down, you'll need to boot into Recovery Console and edit the registry from the command line.

From the command line, you could do it this way:

reg delete HKLM\System\CurrentControlSet\Control\Session Manager\BootExecute /va

Which will delete everything under BootExecute, in this case the convert entry.

When you boot normally, you should put the autocheck entry back via Regedit.