As far as I see MSDOS is still supported, although there's only a virtual msdos, or a msdos emulation that may be based on 32 or 64 bits wich doesn't really matter.
real 16Bit dlls and apps are supported by vista 32bit that contains the 16bit emulation. In vista 64 bit this is missing.
In case you need it:
You may still use virtualpc: http://www.microsoft.com/windows/virtualpc/default.mspx
Or any other virtual machine that is supported by your hardware and allows to run 16 bit apps.
real 16Bit dlls and apps are supported by vista 32bit that contains the 16bit emulation. In vista 64 bit this is missing.
In case you need it:
You may still use virtualpc: http://www.microsoft.com/windows/virtualpc/default.mspx
Or any other virtual machine that is supported by your hardware and allows to run 16 bit apps.