[dba-VB] License Keys

To all,

I have been unemployed for around 12 months now and been trying to teach
myself vb.net, c#, asp.net, php etc....I am at a stage where I am thinking
of going self-employed and I have a few software ideas, however I have
never come across how to put license keys onto an application.  could
anyone point me in the direction of how to do this, so that a user can have
the software for about 30 days, but then if not entered a license key it
will not work etc.

Thanks in advance for any help.

-- 
Paul Hartland
paul.hartland at googlemail.com