How do I install my Windows 95 on my external hard drive?

I have a copy of Windows 95 and I have a portable hard drive. I tried entering its' setup.exe but it wont work. I have Windows Vista now. It says, "Setup cannot run from within Windows NT. You need to shut down Windows NT, and then start MS-DOS. If you have a previous version of Windows, start that version then run Setup. Otherwise, run Setup from MS-DOS." How do I install it? I want to install it on my portable hard drive.