Alot of people when they hear the word server they think of a giant computer. This is false. Any computer can be a server. It doesn't matter what the specs are or anything. In fact some people even have servers that have 486 CPU, 64KB RAM, 512MB HDD. Yes I know it's old and slow as hell. But I'm bringing this point up for one reason. A server doesn't have to be very powerful. It's just that it works best if it is.
For example one of my Servers is 4 core CPU, 4GB RAM, 1TB HDD. The cardinal rule is the more money you spend on hardware the better server you will have. And this is why most people think of huge computers when they think of a server.
But what really make a server is not the hardware but the software. For instance I could have the state of the art hardware, but if I'm running Windows XP Home or some other OS with no server software then the computer will just be a computer. It is the software that makes the difference.
So if you have computer you want to use as a server your good to read on. Now you need to decide what your going to want your server to do. Do you want to be an ISP. Or do you want to Host websites. What about being a game server. Or maybe you want to host E-Mails. You need to decide this for the next step is going to focus around this decision.
Okay lets say you wanted to make a game server. You basically just need to have an XBox 360 and host your own game room. Which is exactly why I'm going to tell you how to do the second easiest server to make.
A Web Hosting Server
So you wanna host some websites. Well now we are getting to the fun stuff. First you have to decide what type of sites you want to host. Are you going to be hosting sites that are made with ASP.Net. If so then you will need to use Windows OS. For any other site you could and I recommend using Linux, even if you can use Windows to host those sites, Linux is a better choice.
So I'm going to start off telling you a little secret. Most people believe you need Windows Server to host ASP.Net websites. This is not true. Windows Server 2003 Web Edition is expensive. But it lets you setup virtual host(multiple websites on with the same IP address). Windows XP Pro also can host websites with the same tool Windows Server uses, this tool is called IIS, but you can't host multiple sites with it on XP. Windows XP Home doesn't even have IIS, so doen't even look for it in Windows.
But if you ask me IIS is POS(Peace Of Shit). If you want to host websites download the latest version of Apache HTTP Now the cool thing about Apache is that theres versions of it for almost all OSes. So if your using Windows download the Windows version. And it will work for all versions of Windows from 98SE to Vista. Not only that but it also supports virtual host.
Now theres already a wealth info on the Apache site on how to use it so I'm not even going to explain how to use it.
Now if your going to host other websites. I would suggest using Linux. Mostly because it's cheaper, more secure, more powerful and more flexible.