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...