How to lock registry Value so it wouldnt be changed without effecting the system ?

You can set permissions for registry keys. Remove Edit permissions from System and Administrators group, then the key should be read only. Alternatively, a quick .reg command in a startup could reset the registry key every time PC starts . . .

Here is a more comprehensive explanation of how to change permissions on a registry Key

Open the Registry Editor, and navigate to the key you want to protect. You can’t protect individual values, but rather only the keys that contain them. This means that if you lock a key to protect one of its values, none of its values can be modified. You can, however, choose whether or not your changes are made to the subkeys of the selected key.

Right-click the key, and select Permissions.

Click Advanced, and then click Add.  If the Add button is disabled (grayed out), you’ll have to take ownership of the key, close the Permissions window, and then reopen it before you can make any changes to the permissions of this object.

In the Enter the object names to select field, type Everyone, and then click OK. (The “Everyone” user encompasses all user accounts, including those used by Windows processes and individual applications when they access the Registry.)

In the next window, “Permission Entry for...”, click the checkbox in the Deny column, next to the actions you want to prohibit.

When you’re done, click OK in each of the three open dialog windows. The change will take effect immediately.