You can integrate .NET Reflector with the Windows Explorer shell so that you can open assemblies in .NET Reflector directly from Explorer.
To do this open up .NET Reflector, then click Tools > Integration Options on the main menu. Now just check the Windows Explorer option at the bottom and hit OK.
Once you’ve done this you should find that in Explorer, if you double-click on a DLL, it will open directly in .NET Reflector. You should also find that there is a Browse with .NET Reflector item on the right-click context menu for DLLs and EXEs.