QUOTE(jg71 @ Jul 1 2008, 10:24 PM)
so, it's legal to install oem version in old pc? how about if my hard disk spoil, then the oem windows can't be used in new hard disk?

what you meant with 'have problem activating the license'? is that will fail to install the oem windows in another pc?
The OEM license is tied to your motherboard. If hard disk spoil, still can use the license. Motherboard dies, the license goes with it.
The OS will not fail to install on another computer, and you probably can still activate it too either online or via phone. But considering that you are using it for business, you will have to buy another license if the computer is broken without warranty.