Your main components are going to be:
Motherboard (big board stuff plugs into)
CPU (processor, it needs a heatsink and fan)
RAM
Video Card (some motherboards have ones built in, generally not as good as seperate cards)
Power Supply (pretty important if you are running mid-high end components)
Hard Drive(s)
CD/DVD Drive(s)
Case
(keyboard, mouse, monitor, but those can be recycled from older systems easily)
I believe that is all you need to get a running system for something like Windows. Arguably you could run without a case, but its not very practical
You can buy all those parts at
www.newegg.com or really any other online store that sells components. Ordering is a little tricky because you need to make sure you get the right processor for your motherboard, and the correct RAM, as well as the right type of video card. It isn't real hard, because really you are just matching up things like DDR2 (RAM), Socket 775 (what the processor will fit into), and PCI-e (video card slot). Those are just examples of things that can be different, not necessarly a recommendation.
Reading the forums is a great way to learn, but it can be overwhelming at times, especially if you don't even know what people are talking about when they say things like 8800 vs 600, or x2 4600 vs E6300. That is where looking through sites like newegg for the components listed above will help, you'll start seeing those numbers on pieces of hardware.
As far as whether you can get what you have to run XP. Its almost certainly answerable as Yes. XP will run on almost anything, it will even run on Pentium 2 computers, and if you turn off a lot of the visual enhancements, it will run pretty nicely for office type stuff.
Vista is a different story, and you won't be able to run Vista on older hardware well if at all. Vista almost requires a system built in the last 2 years for it to run nicely.
