Start from scratch, reusing as little as possible from your old system.
You'll need a new motherboard (I like Asus) and processor (w/ heatsink/fan.) You'll need new RAM to match the requirements of the mobo.
Buy a new hard drive and format it from scratch, then install the operating system (WinXP, probably,) and all your apps. Install your current hard drive as a second drive, so you won't lose any of your data files.
Then give your old system to a deserving charity that will appreciate getting it.
Do a Google search for "installing motherboard" and read a few of the guides available online. Lots of good tips to help you do a good job first time out of the box.
Get the best prices at http://pricewatch.com
Agreed. I built my own system a few months ago, keeping only the old HD and monitor, and it destroys any store-bought box I've ever owned - not to mention being cheaper.