If you are creating application that works with registry (e.g. your own registry editor) you are probably aware that 64-bit and 32-bit applications do not necessarily share registry keys. If you have 32-bit application your view of registry will not match one that 64-bit application gets.
.NET Framework 4.0 helps here a little. There is RegistryView enumeration available so code can specify which “bit-ness” of registry is desired. It works both ways: 32-bit application can specify 64-bit view and 64-bit application can specify 32-bit view.
As always, you cannot get what is not there. If you have 32-bit Windows, specifying 64-bit view will still give you 32-bit one.