Avalaunch should have a FTP client built into it.
You should be able to go into your avalaunch network settings to see the IP address, user name, etc to FTP from your PC to your xbox. Any reasonable FTP client on the PC will do. You may also have to configure your IP address to be on the same subnet as the xbox. (i.e. windows internal IPs are usually 192.168.x.x, I think avalaunch might give you a 10.0.0.x number, these need to be in the same range)
I'm not real familiar with flash fxp or qwix... are these FTP programs for the xbox? If that's the case, just hop into your front end and use a FTP client on your PC to transfer the files over to the xbox rather than the other way around.
I have a mac and use RBrowser as my FTP client on the mac, connecting to the built in FTP client in unleashX.
Good luck.