You need FTP server software. There's several of them available, check Tucows for example.
Yes, there's some risk involved. If you're not careful people will start to use your harddisk to store illegal content (child porn, pirated software, etc. etc.) that they don't want on their own machines.
So you'd better be careful about how you set permissions on your server.

Your ISP may also not allow you to host an ftp server (or any server) on your connection, in which case doing what you want can get your account suspended.

As to how to get the laptop seen outside the LAN, that's a question of DNS record propagation which I'm not familiar with (not being a network admin). Not even sure you can do that yourself without the help of your ISP.

The ip would have to be my Host computer's ip with a redirect port, not sure how to find it out though

I will check out some of the ftp software, and hopefully set up my own mini service thanks for the help

Also I will post if any major problems occur ^^

i have no idea how create my own ftp site ,to access my files from other computers.please tell me the process step by step.on windows xp.