I need some information on enabling-disabling a software (registration-activation)

[C# Programming]

I have a project in C#. Now i have this problem: after installing this software it should be disabled and the user must register it in our website, then with an activation key that would be given in the website the software should be enabled again!
I really know nothing about how it should be done.
Would anybody please help me???
it's so urgent!!!

Roshanak