Seems to be best supported right now (package in Debian, optional drivers for Windows, starting unmodified VMWare machines ‑‑ after you guess right settings that is!)
Links
• Windows ACPI problem  ‑ agp440.sys problem
• How to migrate existing Windows installations to VirtualBox  ‑ it's applicable to other emulators as well!
• Creating and Configuring Headless VMs in VirtualBox 
• vbox‑tools  set of shell scripts to manage VirtualBox
Debian Install
OSE version (no USB!) comes in Debian, compile vboxdrv with:
root@llin:~# module-assistant a-i virtualbox-ose

Command line VM creation
VBoxManage createvm -name "VirtWorkshop" -register
VBoxManage modifyvm VirtWorkshop -memory 512Mb -acpi on -boot1 dvd -nic1 nat -dvd /rest/iso/ScummVM\ Launcher\ 2.iso 
VBoxManage createvdi -filename hda-8Gb.vdi -size 8Mb -register
VBoxManage modifyvm VirtWorkshop -hda hda-8Gb.vdi
VBoxHeadless -startvm VirtWorkshop

Convert .vdi disk to raw image
sic this requires more than reading ‑h output:
dpavlin@x200:/virtual/win$ vboxmanage internalcommands converttoraw hda-winxp.vdi hda.img
VirtualBox Command Line Management Interface Version 2.1.4_OSE
(C) 2005-2009 Sun Microsystems, Inc.
All rights reserved.

Converting image "hda-winxp.vdi" with size 8589934592 bytes (8192MB) to raw...