intel d845wn
The usual steps would be to hook up a monitor, mouse, and keyboard. Then power up to see if it works. The machine should go through POST, and then look for an operating system on the hard drive.
A machine this old that would probably be Windows 98SE, or Windows XP Home Edition, or perhaps (heaven forbid) Windows ME. If no OS can be found, then you're going to have to get one and install it - that probably will mean you need a CD ROM player also installed.