[R@f]

If you want to turn your PC into a simple server, just install a server OS on it like Windows Server or a GNU/Linux distribution like Debian or Red Hat. You can also use a "normal" Windows but you won't have as much tools as Windows Server.

If you want to have multiple VPS on a single PC, you need to get a virtualization software or OS. It depends what you want to do. As written in the wikipedia article that DaRealShazz gave you, you have two types of virtualization :

• Bare meta : Install a virtualization dedicated operating system (hypervisor) on your PC. I worked with three different ones (VMWare ESX, Citrix XenServer, Microsoft Hyper-V). You can get ESXi or XenServer, which are amazing products, for free with some limitations (some features for big infrastructures you won't even use). If you have a PC you want totally dedicate to that, go with ESXi which is very easy to use, you just need to fill out a form in order to get the free licence.
• Hosted : Install a virtualization software like Oracle VirtualBox or VMWare Workstation on your current Windows installation and create your VMs with that. This is more recommended for tests, except if you use OpenVZ on a GNU/Linux host.

Image source Wikipedia