Software Activation Methods ??

Hello,

I am looking for a license component which I can easily integrate in my
existing C-applications.
I would like to offer trial-versions of my applications via my website for
potential clients to download.
So the purpose of this license component would be to control which features
of the application should
be disabled after the trial period has expired.

The potential clients can use the application will all its features for a
30-day trial period after which selected
features of the application will be limited/disabled.

If the client decides that he would like to buy the application, he can go
to my website and fill out and submit
a form. The client pays for the product and a license key will be generated
automatically. The client enters the
license key in the application to enable the features that he paid for.

My questions are:

1) Which license generator should I use on my website to automatically
generate license keys?
2) Which license component would you recommend that I use?
3) Other advice?

If you have several suggestions, please post them.

To sum up what I am looking for:

1) The license component must be easy to integrate into my existing
C-applications
2) It must be easy to register new products in the license generator
3) The license key has to be "secure" and hard to "decode"
4) The license generator must also be secure in the sense that it shouldn't
be easy
to figure out how the license keys are generated
5) It has to be close to impossible to reset the trial period for the trial
versions
clients download from my website.

I look forward to receiving some advice and suggestions.

Thank you in advance.