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The banks do it in a following way:
- the first time you login, they save some unique identifier related to your computer and OS.
- if you log in from another computer (or reinstall your Windows), they will ask you some security questions that you've provided answer to before (e.g. name of your best friends from high-school or name of your first pet)
- once they check out, you'll get access, and they save the new ID
- I am sure that if you go back and forth, it will raise a red flag and they will investigate it.
Maybe somebody could design something similar for XOOPS - it would probably solve your issue.