Check first if your motherboard allow booting from USB. If it does (like booting from an USB floopy drive), I don't see why it can't boot from an exteral HD enclosure. You'll just need to make an exact image of the HD in that system, dump to the enclosure HD. This can be done in Windows or in DOS using Ghost or other software. You might need to run SysRep first before imaging due to change in hardware profile, thus also the SID (unless your imaging software was smart and took care of that for you)