Here's the deal, Lil homey from accross the sea:
Your friend is either messing with your mind, Or he has indeed exploited an ftp service on your machine. Which as Acid stated, is unlikey - but possible.
Soggy's advice is good, but there is an easier way to monitor connections to your machine over the network. - Open a command prompt and type "netstat"
"netstat -n" gives you the IP's and netstat -n 90 will re-list it every 90 seconds.....
Word of caution - this will give you a lot of info if you are surfing the net, using ICQ etc.
It also tells you what port the connection is on - You see where I am going with this?

Remember - lamers want you to "Ph33r" them - for instance, I recently had someone claim to have "taken an image" of my HD while in #antionline - Sure sounds "733t" don't it? - Well, what this fool didn't realize is that there are probably 30 or so computers connected to a router - All of which would appear to have the same IP to the outside world - Sure hope he imaged the right HD - Don't you? - I have been looking for him to ask him to show me the image - can you guess wether or not he will?
Technical Note: It is theoretically possible to differentiate my packets from the others by MAC Address, However, until I see some kind of proof with my own eyes, I am calling it a pathetic attempt to gain my respect.