Is it possible to get hardware information of clients' PCs from asp.net?

I have a site, users need register accounts to login. How can I code it, so that every time only one instance of the user can login the account?

For example, one user (AAA) created the account, he tells another guy (BBB) his login. So, AAA and BBB can login at the same time. How can I prevent this happen?


thanks

bear