What you're talking about is a chain-loader but if the idea is to be
able to use the USB device then be able to install software and use
this PC then your better option would be to fit a hard disk IMHO.
Presumably it'll only have IDE ports but these drives are still around
even though SATA is the more prevalent now ( and SSD's the 'New Kid on
the Block' ). Even second hand drives abound if funds are an issue,
even if it's not being worth spending much on an old system.