A good place for parts is Newegg.com. I would suggest an AMD system for price considerations, more bang for buck. Check them out you should be able to build a good system for under$1000 easy.
You mentioned that you were going to use Windows Pro from another machine. I think this might not be a good idea. When you register and activate the OS, yes they make you do this on ALL installations, Microsoft keeps records of which machines have which serial number. You can buy a licence for a second machine in your home, Or just by a new copy. I think you would be unwise to try to setup the OS on two machines.
If you do not want to build the machine yourself, it is not that hard. (See the sections on CNET) find someone in your area who builds them. Computer Clubs and Computer Labs at schools can be a good resource. Remember Computer are machines to run software. You will find that your software costs are usually more than your hardware costs.
I am a looking to order a custom built computer from one of the web sites that offer these. I know I want a Pentium 4 or similar 3.0 Ghz (or less if OK for what I need), an ATI 128mb graphics card or similar, dvd/52X32X52 CDRW, dial up modem, sound card. No Monitor, keyboard and mouse OK but not necessary. I have a windows XP professional from another computer, probably need some sort of microsoft software bundle. I am totally clueless on all the technical aspects...This will be used by my kids for games like Roller Coaster Tycoon 3, etc, not major gamers but we do want something that will work, for music downloads, cd burning, homework. My other computer is fine for me. What can somebody out there recommend? I was looking at Tigerdirect.com, cyberpowerinc, or Puget? Many thanks in advance! Oh, the cheaper the better!!!