As was already said, running a web server or an FTP server will do it just fine. But remember that anyone who can access your computer can also exploit it. Windows is full of security holes, and by exposing your computer to the internet, you're opening the door for hackers and exploit worms like Code Red.

Is your computer connected via dial-up or broadband? If the latter, what security is currently in place between your computer and the rest of the world? (Maybe it's a moot point and you've been long since compromised.)