I am developing a Windows Application. That requires users to register to use it...
Now, I am storing my license info as a file in APpData. But deleting that file resets the trial version date. So, I am now planning to save it in registry.
But, Most of the Users will not have administrative privileges (Limited Users) in Windows to Access the registry.
What can I do ? Where can I save my serial number and date ?
In my opinion the point is you have to change how you manage your license. I wrote a project (hosted on git) to demonstrate some of techniques described here.
If they delete license data file then trial restarts? Do not start application if file doesn't exist and create it with an install action first time it's installed.
Now you face a second problem: what if they uninstall and reinstall application? Second step is to move this file to application data folder (for example Environment.SpecialFolder.CommonApplicationData). This is just little bit more safe (because application data won't be deleted when uninstall) but it's still possible for them to manually find and delete it. If application will be installed by low privileges users there isn't much you can do (you can't try to hide license somewhere in Registry).
Now it's a game between you and crackers. They'll win, always. You'll only make life of legitimate users more hard so read cum grano salis. Where you may store license data:
\0in its name. Take a look to this nice post.
How to hide license in a file?
If you're going with a file (it doesn't matter where it's located) you may consider to make crackers life (little bit) harder. Two solutions come to my mind now:
Alternate Data Streams. File is attached to another file and they won't see it with just a search in Windows Explorer. Of course there are utilities to manage them but at least they have to explictly search for it.
Hide it inside application data (a bitmap, for example, using steganography). They just don't know it's license data, what's more safe? Problem is they can easy decompile your C# program to see what you do (see paragraph about Code Obfuscation).
Probably many others (fantasy here is our master) but don't forget...crackers will find it (if they really want) so you have to balance your effort.
Keeping your license schema you're now on a dead path. Decision you have to take is if the risk they use trial longer than allowed is higher than risk they stop to use your application because of boring protection.
If you can assume they have a network connection then you may validate license on-line (only first time they run your application) using some unique ID (even if it's about Windows 8 you may take a look to this post here on SO). Server side validation can be pretty tricky (if you want to do it in the right way), in this post is explained an example of program flow to manage that in a proper way.
Your license file/data is now in a safe place. Hardly crackers will find it. Now you need another step: obfuscation. If your license data is in plain text once they found your file it's too easy to change it. You have some options (ordered by increased security and complexity):
Please note that data obfuscation/encryption can be used in conjunction with above mentioned steganography (for example to hide encrypted license file inside an image).
If you're not using license signing with asymmetric encryption then last step is to obfuscate your code. Whatever you will do they'll be able to see your code, check your algorithm and workaround it. So sad, you're deploying instructions manual! Obfuscate with an Obfuscator if you want but what I strongly suggest is to move your license check in a less obvious place.
I like this method because they'll see there is a license check but...they won't find license code. Of course any good cracker will solve this issue in 10 minutes but you'll be (little bit more) safe from random ones.
To summarize a little bit this is a list of what you may do to provide a stronger license check (you can skip one or more steps, of course, but this will reduce safety):
Addendum: Software Protection Dongles
A small addendum about hardware keys (Software protection dongles). They're an invaluable tool to protect your software but you have to design your protection even more carefully. You can assume hardware itself is highly secure but weak points are its connection with computer and communication with your software.
Imagine to simply store your license into the key, a cracker may use an external USB (assuming your SPD is USB) to share same key with multiple computers. You should also store some hardware unique ID within the key but in this case weak point is connection (hardware can be emulated by a software driver). It's a pretty easy crack and this false sense of security ("I'm using Software Protection Dongle, my software is then safe") will make your application even more vulnerable (because you risk to forget other basic protections to simplify license management).
Cost vs benefits for a poor designed protection using SPD should make you consider to use a normal USB pen drive. It costs 1 $ instead of 15/20$ (or much more) for a SPD and you have same level of protection against casual crackers. Of course it won't stop a serious cracker but also a poor designed SPD won't stop it.
A true protection (assuming you're not running on a DRM enabled device) is a dongle which can also execute your code. If you can move some basic algorithms (at least to decrypt vital - and dynamic - support files) into the key then to crack your software they will need to crack hardware. For a half-decent dongle this is a very very very hard task. More carefully you design this and more code you move into the key and more you'll be safe.
In any case you should doubt about marketing campaigns: software protection with a dongle isn't easier. It can be (much) more safe but it isn't as easy as vendors say. In my opinion plug-n-play protection cost is too high compared to its benefits (benefits = how much it'll make crackers' life harder).