Cant Download Updates

I have 3 computers (2 laptop wireless, 1 desktop wired, all XP Pro) all 3 will not download updates for Windows, anti-virus, anti-malwre, even new programs. I can visit all websites fine and computers are not slowed. The only exception is applications on facebook (farmville, cafe world, etc...not something I use, BTW) which will not load. In each instance, the download will begin but stop after a few seconds and just sit. Sometimes I will get "connection to server was interrupted."

First thought was virus/trojan/worm but all scans came up with nothing so I did a clean install of XP on my desktop. Same problem as soon as I got connected. I took the laptops to a friends network and it worked perfect. Fast, updated everything, all apps work on facebook. So I think it has to be my internet. I call AT&T the run all kinds of test and come up with nothing assure me it is either my computer or modem.

I switch modems with my friend, whose network my laptops worked on, but it still does the exact same thing. I am at a loss...anyone with any ideas?