If you have windows, and install Internet Information Service ftp will be set up automatically, along with many other ways to access your machine. I don't recommend it.
The best and easiest way to do this is to install Ubuntu as a secondary operation system, or run it from a CD during the times when you decide to allow your machine to function as a server.
If you use the server edition of ubuntu, ssh will be installed you'll need to create users.
They can then use filezilla, which is free, and access your site just like an "ordinary" ftp site, except in the box that says "port number" you would enter "22".
If you choose the desktop edition, you can install it on your hard drive inside of your windows installation, with a few mouse clicks from the standard installer. But you'lll have to use the package manager to install ssh. It is much easier to create additional users with the desktop edition.
Short answer is: if you allow this in windows, your machine will be hacked.
Addition info: don't allow anonymous ftp, create users who have a specific folder available to them.
If you have a Macintosh, you can install sftp and add users in a manner similar to Ubuntu, thanks to the linux/bsd underpinnings of OSX.
If you choose the Ubuntu route, complete instructions are at http://www.ubuntu.com/
get the 8.04 version.