[Baliw]

Option 2 is your best bet. There are several hardware ID's you can use. Network MAC address is probably the least likely to change.

If you can't leave anything at all on the system then you have to use something that's already on the system that doesn't change as a key.

[caw]

One caveat is that MAC addresses are not guaranteed to be globally unique. In practice they may be, but with sufficient amounts of similar hardware you can get duplicates.

Use the MAC if you're looking to make it silent. Use the free registration if you also want to harvest emails for your marketing people.