unable to download large files. download just stops in middle/pauses

I would really appreciate any help I can get. The problem that has started over the past couple months is that I cannot download large files, anything over a couple hundred MB's say. For example, today I tried downloading 3d mark 06. Firefox says it is at 59%. After that, its just at a standstill. It did that on my other system, and this system is brand new. Is Vista causing this, or is my ISP (who charges $70 a month for this slow crap DSL 256k down) blocking my downloads? I'm not downloading anyhting illegal. It just sucks I can't download any game demos or large programs.

Hey i have the same problem but if i am trying to play games like Combat arms or War rock it will log me off or say that someone else is using my profile.
But if i try to download a file like anything bigger then 50MB it will stop in the middle like at 70%.
And by the way its not my ISP Or router because i have another PC That use's the same router and can download just fine.

Any help would be appreciated, thanks.