Each license key is good for one PC. So your retail copy is already used and can't be used again unless you uninstall it from the build it is currently on.
You can use an OEM version, but keep in mind that the OEM versions are tied to the motherboard. If the motherboard dies and you can't find the same model calling microsoft will likely get them to reactivate it. Even if they don't you can buy two OEM versions for the same price as a retail version.
Your final option is to buy a Windows 7 upgrade kit. While also cheap (I think it is also $100) it requires that the PC have a previous version of windows installed before you can install it.