Registry.GetValue always return null

if you are using 64 bit operating system, when you are trying to get HKEY_LOCAL_MACHINE\SOFTWARE\RSA it is actually looking for HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\RSA that is why you get null

look at the security permissions on the registry key with regedt32.exe; check if you are running as admin and have UAC turned off. According to the opensubkey documentation it needs to be opened first before accessing any keys; http://msdn.microsoft.com/en-us/library/z9f66s0a.aspx

You don't access the HKEY_LOCAL_MACHINE hive the same way you do in C# as you would in batch scripting. You call Registry.LocalMachine, as such:

        RegistryKey myKey = Registry.LocalMachine.OpenSubKey( @"Software\RSA", false);
        String value = (String)myKey.GetValue("WebExControlManagerPth");

        if (!String.IsNullOrEmpty(value))
        {
            ProcessAsUser.Launch(ToString());
        }

Update:

If it returns null, set your build architecture to Any CPU. The operating system may virtualize 32-bit and 64-bit registries differently. See: http://msdn.microsoft.com/en-us/library/windows/desktop/aa965884%28v=vs.85%29.aspx, Reading 64bit Registry from a 32bit application, and http://msdn.microsoft.com/en-us/library/windows/desktop/ms724072%28v=vs.85%29.aspx.

The statement of Jason is right, the operating system is the problem, the below code will help you to resolve.

RegistryKey localKey;
if(Environment.Is64BitOperatingSystem)
    localKey = RegistryKey.OpenBaseKey(RegistryHive.LocalMachine, RegistryView.Registry64);
else
    localKey = RegistryKey.OpenBaseKey(RegistryHive.LocalMachine, RegistryView.Registry32);

string value = localKey.OpenSubKey("RSA").GetValue("WebExControlManagerPth").ToString();