I recently tested qemu, an open source processor emulator. qemu runs on various host platforms and can emulate various target platforms. They also provide an accelerator module which runs as kernel module on x86 based Linux hosts and enables native instruction execution when an x86 based system emulates another x86 based system. I successfully installed Windows XP on a Linux host, and was really surprised about the performance of the emulated system! It is of course not as fast as a native installation, and especially graphics operations appear to be rather slow (which might be caused by a poor graphics card emulation), but at least I am now able to use the few MS-Windows applications which I still need.