How do I mount an ISO file to an external hard drive so that I can boot from it?

I am trying to upgrade from Vista Home Premium to Windows 7 Ultimate using my external hard drive. Copying the iso image to it then booting resulted in an "NTLDR is missing" error, so I assume it wasn't "mounted" properly so that I could boot from it. How do I mount it properly? Ideally I won't have to buy any programs to do this. Thanks for your feedback.