Corrupt Downloads on Entire Network

Alright, first of all I'd like to say that I've heard that Majorgeeks.com is an absolutely stalwart website with fantastic forum support. I've been asking everywhere and talking to everyone about my problem, and so I hope you all can help me out.

My problem is this... Every time I try to download a file that is an executable on any computer on my wireless network (such as ad-aware, driver downloads from Nvidia.com, or Spybot S&D), the download is said to be corrupt. Specifically, I'm able to download the file, but when I try to install it, it claims that "Corrupt installation detected. Check source media or re-download." Previous to that, it would say that the "Executable you are trying to run has been corrupted." Now I thought this may be a virus or malware of some sort messing everything up, and so I scanned with Norton anti-virus, and found nothing. I also tried using IE7 and Firefox, clearing out my cache, running a scan disk... everything. Still the same problem persists.

I tried running downstairs to my computer in the living room, running on the same wireless router, with the same kind of wireless NIC (a linksys Wireless-G). The same error occured. I also tried on my older computer that has an older NIC in it, I can't remember what kind of card, but it required the standard CAT5 cable, which I hooked into my wireless router. Same issue occured. I read some other forums and people claimed it was a memory issue. I thought, "On all three computers at once?" Someone else said that it was a virus. Same issue, "all three computers at once?" Then someone said "Maybe it's your NIC's or router?" Now maybe it is the NIC's, but all three of them suddenly causing this problem... I don't think so. Someone claimed that this was a network issue, and so I thought maybe Majorgeeks could help me out.

So what do you fellas say? Anything you might be able to do for me? I'm willing to do anything to get this working!