In my current Windows Vista install (as well as other Window's installs), I can share files by sharing folders and having public directories. What's the difference between having an actual file server and having a computer that just shares files?
People typically define a file server as a machine that is designed purely for sharing files. Such a machine could be anything, but generally a good file server should aspire to have the following features:
Such a machine does not need any form of graphical interface (therefore no graphics card or even monitor).
A regular computer does not generally have things like automated backup or even a UPS, and power consumption is much higher because of all of the extra things a regular computer does - graphics, sound, and CPU utilization. etc.