LogFAQs > #883803566

If you install Windows 7 multiple times on different hardware configurations with the same key eventually Microsoft will mark the key as invalid and it will no longer activate. For a legal install at that point you'd either need to complain to Microsoft or get a new key.

So unless you're installing Windows 7 multiple times on the exact same computer, you may not be able to install Windows 7 as many times as you like with the same key (I think it's more forgiving for some hardware changes than others, and if you're using entirely different PCs you'd definitely get marked eventually).