We'll be using apache: very popular, free, open source server software.
While making your computer into a server will be very quick, you should read through the Apache http server software documentation to learn about how to set it up securely (so that people don't hack into your computer). I'll give this advice but read elsewhere for more:
1) best to install this server software on an old computer that you don't use for anything other than as a server.
2) best to create a seperate user account in windows with limited system access and install this software in that account.
If you want to do this on linux instead of windows (more secure, slightly more complicated) the best combination would be puppy linux and xamp for the server software. Puppy linux can be loaded off a usb jump drive or a CD, so you don't need to uninstall windows, just boot the computer with one of those. Here's easy setup videos for puppy linux: http://rhinoweb.us/
See this instructable for how to install xamp once you have puppy linux (or some other linux) working:
Step 1: Download Apache Server Software
_Or_ go to http://www.apache.org/, navigate to the download from mirrors page, pick a mirror and download from there. As you can see from the above link, you'll want to navigate to the mirror's apache, httpd, binaries, win32 folder.
What To Download:
= You should download the newest version (highest version number), it's 2.2.6 as I type this. Check here to see what the newest version is if you want: http://httpd.apache.org/
= You want the .msi file, this is a windows installation file (like an .exe)