I am almost computer illiterate when it comes to the internal parts. I have three computers and would like to combine the parts to make one well equipped one. I would like to install Windows XP from one computer into another. the only parts I know are the memory cards and CD Roms.
The operating system is installed onto the hard drive (HDD) and runs on the central processing unit (CPU). It accesses internal memory chips called random access memory (RAM) the chips for which come on single in-line memory modules (SIMMs) or dual in-line memory modules (DIMMs). The visual output comes from a either a graphics chip on the motherboard (which fundamentally connects all these parts together) or a separate graphics card which fits into an expansion slot on the motherboard.
You need an optical drive - either CD or better still DVD - though they are becoming less important as most software can be downloaded from the Internet.
What you need to do is find the specifications of each of the parts to work out which ones are "best". However, not all parts are compatible with each other. You might have a fast CPU but it won't fit your motherboard for example, or your largest hard drive has the wrong connections (SATA vs IDE).
Another source of incompatibility could be the graphics card. Modern cards use what's called the PCIe expansion slot while older one use an AGP slot. Even older ones use a PCI slot.
I'm sorry if this sounds confusing, but there's a lot of acronyms and jargon which I've only scratched the surface of.
If you aren't confident of your abilities or are unsure of what you have then it might be simpler to take the parts to a local computer shop. Ask around your neighbourhood first, but you should be able to find one that won't rip you off. Ask for a quote on how much they'll charge for building you a machine out of your parts. If you have any doubts say you need to think about it and post another question here for more advice.
Once you have your machine you should be able to reinstall Windows onto that machine. There may be activation issues (covered in questions here on Super User) but a call to Microsoft can sort this out.
To keep the answer simple, Almost all modern computers have a hard drive (or SSD which behaves like a fast hard drive). Windows is invariably kept on the hard drive.
You should be aware that you may not be able to just take the hard drive out of one machine and put it in another - while it could work, Windows requires bits specific to your computer type (software drivers) to make it work. If some important drivers are missing it will not load.