When you have a router, you need to forward port 20-21 (or the port where you're running your ftp, but these are the defaults) to your pc. But first of all, you need to test within your network to see if the server works.
You also can't have a dynamic IP (from ISP and within network).
I can't tell you exactly how to port forward, bc it's different on every router, but when you go into the configuration of your router, you need to search for port forward. Just set the ports 20 and 21 to be forwarded to the IP of your computer WITHIN THE NETWORK (192.168.2.x most probabaly in you case, where x is a number). Then, when you want other people from outside the network to access your ftp, you need to give them your IP from outside the network (
you can check this here). When you have a dynamic IP, you need to setup a no-ip account (or something similar, if you need help with this, just ask, but not going to explain now).
Hope this helps.
PS: when you're running WinXP (only Pro I think), you don't really need any software, bc Windows can set this up. But it's better to install software for this.
Greetz Johan