Corrupted Downloads

[SuperFun]

When I download files (EXE and MSI) from my Bluehost hosted website to any one of my three home computers using IE7 or Firefox 2.0 the digital signature is broken, indicating a corrupted download. Ignoring this and executing the file anyways results in errors, either "The cabinet file X is corrupt..." or a WinZip error.

Files that formerly downloaded fine are now corrupt.

Files downloaded from other websites are fine.

Downloading these same files using UnlimitedFTP, the generic Windows ftp program, or the Bluehost File Manager works--no corruption.

Almost all other computers can download my Bluehost hosted files just fine--all other computers that I have tested are fine. So far two customers have reported corrupted downloads.

My three home computers all run McAfee SecurityCenter 8.0. Each computer has a different OS: XP Home, XP Pro, and Vista Home Premium.

The most recent customer to have this problem had Trend Micro PC-cillin Internet Security. FTP worked for them on the first try.

Before the most recent customer's problem, it seemed that McAfee was the common thread but disabling it still resulted in corrupted downloads. Also, the fact that downloads from other websites are OK contradicts this, as well as the most recent customer not using McAfee.

All files in this discussion were properly uploaded using binary form. They are obviously not corrupt on the server since I can download them in some ways and so can other people.

I have flushed my cache, disabled McAfee, tried at different times of day, connected directly to the Internet without a router, and set my website as a trusted site under Internet options, all to no avail.

I am at my wits end. I can find no useful suggestions anywhere.

Two last facts:
1) A corrupted file and its original both have the exact same length, down to the byte.
2) The files we're talking about are about 3MB.

[SuperFun]

In an effort to better track down the source of the corruption, I took one of the laptops to a neighbor's house who just happened to have the same ISP. The downloads were fine on his computer (exonerating the ISP). The downloads were also fine on the laptop (exonerating the computer).

Back home, I tried a download directly through the modem, circumventing the router, and it worked (exonerating the modem). I thought I had tried that before but apparently I messed up.

The ultimate problem was the SPI firewall in the router. As I understand from reading posts elsewhere, some SPI firewalls can replace data within packets (not just in the headers) when they think such data is an IP address they are looking for. If your packet is unlucky enough to have the right byte sequence then it will be corrupted.

I disabled the SPI firewall and everything works fine.

I have a Netgear WGR614v6 wireless router.