There are a few way to do this.
as the first reply said you can partition your harddrive and install windows, then on another partition install an OS like linux, (or another version of windows).
you'll get a menu when you start the computer asking what OS you want to boot into, in this way you have multiple opperating systems and use onoly one at any time.
another way of using multiple opperating systems is to use VMware or microsoft virtual machine.
Personally I recommend VMware over MS virtual machine.
it's a free download.
you run this on our computer, then you click to add new virtual machines to your computer, in this way your computer acts as a host opperating system to other environments, (the only limitation being you hardware).
VMWare allows you to run windows and linux n the same box.
an advantage that vmware has over MS virtual machine is the networking.
you can have your virutaul machines (VMs) running sharing the host PCs network adapter (each having their own address on the network) or you can give your VMs no network adapters), or multiple network adapters,
also you can give your VMs their own private network so that the VMs can communicate with each other but not machines outside of your host machine.
it's a fairly straight forward installation.
the only thing that MSVM does that is slightly better than VM ware is the way in which the virtual hard disks are setup.
in MSVM you create a disk and that disk grows to the maximum size allowed by your hosts hard drive.
in VMware you create a disk of a set size, (eg 8GB) and that will create an 8GB drive size. the VMs on the VMware box can only use that 8GB.
both will let you pass through the USB connections and serial connections on the machine as well as the CD drive (vmware also lets you mount ISO images as the CD drive).
both let you manage the amount of ram used by the VMs quite nicely as well.