I was using Firefox 8.0 (Vista Home Premium, 64-bit). I was getting downloaded file sizes about 38K. I also tried a command-line HTTP get util and got a file size of 46M, but still couldn't open it.
I just tried with IE 9.0.8112.16421 and got a valid zip (46M file size, but a few hundred K less than I got with the command-line util).
The problem is probably with my network connection (HughesNet satellite, which is VERY unreliable)...but you might want to check your sever logs and check the total bytes sent on recent downloads. Let me know if you need more info (i.e., my IP address) to help identify my downloads from others'.
Thanx.