Hi,
Why don’t you just go SMB (Windows Network Share), it’s way better and less fiddly then setting up your own ftp server. You can take a look here if it helps.
http://www.wiimc.org/forum/viewtopic.php?f=4&t=1604
There also is a sticky somewhere about setting up WiiMC with SMB, but the above link has some specific Win7 settings that you might find useful. If you REALLY want to go the ftp server route, Filezilla is a solid product, open source and good support. Just a heads up, you will be fiddling with it for a while to get it setup exactly the way you want (I have way to much experience with Filezilla, to the point of avoidance now!). SMB is much slicker and IMO more stable and uses less computer resources.