Do not let comcast know your running a server they will turn your service off its not to hard to set up a ftp server you need to open up port 21 on your router and on your machine if you are running a fire fall if you have xp pro you can turn on IIS and enable the ftp right from windows or you can down load. Download
FileZilla Server wich is not hard to use. http://filezilla-project.org/
Also if you need a free reverse dns and a freindly name to go with your ip you can go to www.no-ip.org I use that on my server. works great.
To set up a ftp server is not to hard to do but I will warn you
Comcast has a issue with servers that why I do not use them I use Embarq dsl they give me stic ip and do not care what I do with my network unlike comcast they turned me off for this.
in the Comcast AUP link to their AUP
http://www.comcast.net/terms/use.jsp
Prohibited uses include, but are not limited to, using the Service, Customer Equipment, or the Comcast Equipment to: . . . run programs, equipment, or servers from the Premises that provide network content or any other services to anyone outside of your Premises LAN (Local Area Network), also commonly referred to as public services or servers. Examples of prohibited services and servers include, but are not limited to, e-mail, Web hosting, file sharing, and proxy services and servers;"