You cannot access the registry for writing without admin rights - this was a deliberate security change made with the release of Vista, as a result of the abuse which the registry got in previous versions, and the damage a rogue program could do. You can access it for read without problems.
I wouldn't use the registry:
Where should I store my data?[
^] explains somewhere easier to access. I'd use a GUID folder name in the Environment.SpecialFolder.CommonApplicationData folder, and store my licence file there - encrypted or more likely hashed.
You can't prevent the user from altering the system clock: that can be done via the BIOS on power up if he really wants to, and the disk containing the operating system hasn't even been spun up to speed yet by the time that occurs!
The only reliable way to prevent the user exceeding the 30 day limit is to require a check with an internet based site to validate a licence key each time. And that gives you other problems which are likely to annoy more real users than prevent any piracy.
I would suggest that you don;t spend too much time on primitive security: think about how muany copies you expect to sell, and the profit on each - then work out how much you will likely lose to piracy. Compare that with the cost of your time, and it doesn't take much before you have spent a lot more than you will save...and annoy one customer with an overzealous security system, and you have lot that customer for ever!