.NET Reflector can only open "managed code" which basically means ".NET code". So you can't use it to disassemble COM DLL files, native C++, classic Visual Basic 6.0 code, etc. The structure of compiled .NET code makes it very convenient, portable, discoverable, verifiable, etc. .NET Reflector takes advantage of this to let you peer into compiled assemblies but decompilers and disassemblers are by no means specific to .NET and have been around as long as compilers have been around.
You can use obfuscators to make the code more difficult to read, but you can't exactly prevent it from being decompiled without also making it unreadable to .NET. There are a handful of products out there (usually expensive) that claim to "link" your managed code application into a native code application, but even if these actually work, a determined person will always find a way.
When it comes to obfuscation however, you get what you pay for. So if your code is so proprietary that you must go to such great lengths to protect it, you should be willing to invest money in a good obfuscator.
However, in my 15 or so years of experience writing code I've realized that being over-protective of your source code is a waste of time and has little benefit. Just trying to read original source code without supporting documentation, comments, etc. can be very difficult to understand. Add to that the senseless variable names that decompilers come up with and the spaghetti code that modern obfuscators create - you probably don't have to worry too much about people stealing your intellectual property.