Qwix doesn't work on some people's computer when running Windows XP SP2. So far the bug has not been fixed and people have had to manually extract the ISOs and the upload with another FTP program. I found another application that will prefrom uploading straight from an ISO like Qwix does.
Here's my post about it: http://forums.xbox-scene.com/index.php?showtopic=312481
Also are you running an Athlon 64 by any chance? SP2 added a new thing called Data Execuation Protection which I told Windows to disable for Qwix but I'm wondering if there is a bug with the disabling or just something that is activated with the processors themselves. This is just speculatino; however, it would be interesting to get a list of the setups people are having problems on.
Athlon 3000+ o/c 3200+
1GB 400MHz DDR
NForce 250GB using the built-in Gigabit Ethernet Connection.
Windows XP SP2, with all current Windows Updates.