I have had trouble downloading from ftp sites for some time. I thought it was a problem with my particular computer, but if I log onto another network (other than the one I have set up for my small business), I can download perfectly. Is this perhaps some setting I have set up incorrectly with my router (Linksys RVS4000)? I am using Vista, but also have the same problem with Window 7.
Checking the TCP window size as suggested by Shivlu is a fine place to start. But if it were the window size setting of your PC then I would expect that you would also have problems when connected to other networks.
Some routers do filtering of traffic. I am not familiar with the RVS4000. But I believe that it is quite likely that there might be some setting on your router which is creating this problem. If you experience the same problem with several computers (mention of both Vista and Windows 7 suggests multiple computers) then I would consider it even more likely that it is some setting on the router.