You will still like the boot camp assistant, as you can download the driver package for your mini through that assistant, those include the graphics chip and sound chip, and, of course, others that need to be installed with Windows.
However, this page may help you with that -
https://support.apple.com/kb/dl1837?locale=en_US
You began by talking about Win XP (probably not, unless you do a VM install of some kind)
Then, you said that you could boot to a Win 7 installer (probably your best choice, if you want to stay with an older system.
Finally, you said that you get a memory error message about Win NT.
You might decide which version you really want to install.
Finally, make sure that you are trying
64-Bit installs of whatever Windows you need. Your 2014 will need a 64-bit Windows install.