I know it's already been said, but I guess I'll repeat it. You can make a home web server using just your regular PC. No, you don't need to purchase special 'Server Software'. Anyone who tries to tell you that is an idiot.
Basically, it depends on what you want to do. If you want to use your computer as a Home Computer/Web Server, you can just use Windows XP (Vista, Linux, etc. Whatever your OS is) and download the Apache software. Next, you're going to have to look up on what your ISP restrictions are. When someone requests your website, they will have to come through Port 80 to your computer. Many ISP (Internet Service Providers) block Port 80 for this very reason. Next, if they do allow incoming connections you're going to have to mess with DNS and Dynamic vs. Static IP Addresses.
Next, if you're going to use this machine as a strict web server, I would recommend loading a version of Linux onto it. Ubuntu Server would be a great version, plus it's free and fairly user friendly. If this is going to be a critical server, maybe a UPS would be a good idea too. Of course, you'll have to check up on your Ports and ISP restrictions just like above.
Any other questions? Just ask.