Windows

If you are building the machine, here is the basic (cheaper too):
Asus Motherboard (Save on anything else, but get a good motherboard)
Get a PIII 550 or 600
Processor Fan and Cabinet Fan
128 MB RAM
CD-Rom
3 1/4 Diskette drive
(2) 15 GB 7200 RPM HD (two for redundancy; mirroring)
Basic Video Drive (check compatibility list or make sure it has NT drivers)
NIC if you're networking
Modem if you connect externally

I figure about $900.00 for the above setup.
Another thing I recomend, isthat if you intend to run other OSs, get a removable HD tray ($15.00), and an extra disk. This way you can swap between, OSs.

Good luck!

I have a notes server running NT 4 sp4 on a pent 75 with 56mb ram. I am planning on upgrading it but for 2 years it has been sufficent.
I would say that any current hareware would work.

what will you be using it primarily for? Will you have other servers running on it? Is it for home or work?

HEloo..

You Need To Install NT4.0..

486 Machine Or Higher with speed 100MHz 32RAM Or More Normal VGA..& EtherNet Card.
That's all..

Take Care

I have a Pentium 75Mhz with 32MB running NT server. I use it as PDC in a testing domain at home with 4 other PC's and this works fine.
In fact, the hardware you need depends on what you want to do with your server, and not only on the OS you want to install on it.