I can answer you in short- but for a true step by step guide you'd have to do some searches.
First off, if you want a full domain, you'd obviously need to register one. Next, you'd have to use a service like DYNdns to forward this domain name to a new one. And finally, you'd have to have either software or your router (some LinkSys ones will do this- DDNS) update your dns service whenever your ip changes.
Once you can be sure that your domain forwards to your IP, you can then set up the ftp server. There are many small ones out there- just do a search. (There are two that I know are freeware- Cerberus and TYPsoft)
If you plan on using the computer as a server *only*, then you may want to set up a full server there (and then set up good security.) Otherwise, the best bet would be to use a port other than 21 (like 8001 or 10006 or any other port that would not normally be scanned for vulnurabilities...) and have your FTP service on that port. This way, when someone types ftp://yourdomain.com:10009 they'll get your ftp server (or you can have your dynamic-static dns service route all calls to your site to the port you specify, thus not requiring them to input the port number.)
Dont forget that once you open yourself to displaying that you are a server of some sort, you will naturally be scanned and possibly attacked. The best way around this is by having a good software firewall (in addition to your routers NAT) or even better, to have a physical firewall that will not respond to pings... Some ways to stay low include: making your IP not pingable; using ports other than the normal ones for you services; having up-to-date service packs and Windows Updates. There are certainly more precautions- just do some searches.
Good luck to you,
Larry
