You simply have to get a new activation code from them (website/phone call). You don't lose the right to install. Remember that CDs can't be written to from a reader, so if you don't change hardware you can install as many times as you want.
The activation code is generated from a combination of main hardware (CPU, motherboard, ?), so supposedly if you change your NIC, you'll be ok. Activation sucks, which is why I'm glad to have an activation-free copy.
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