I have a windows application that I'd like to sell eventually. What's the
simplest way to provide a secure licensing mechanism ? I'd prefer to have a
licence key that can't simply be distributed across the pirate community
(eg. astalavista) that will then work on any pc, so I was thinking of
somehow tying it to the machine (some unique ID like the NIC number, but I
realise some computers without a network card don't have one!). I'd need to
store a record of the licence keys provided by e-mail address/machine ID so
that I can provide a new licence key to paying customers who move to a new
machine. I'd also like to build in an expiry date if possible.

Anyone have any good advice on this ?