Download unreliable with certain programs

Hi there!

I have a slight problem with downloading.

When I use download managers like FlashGet or GetRight, or even just normal browsers to download... the file always ends up being currupted.

It's very weird because when I download with Kazza... the file is always pretty much 100% reliable...
But when I use other programs like FlashGet or Getright... corruption happens so much that I have lost all hope in any of the downloads.

I don’t think the files would be 'originally' corrupted because firstly, this happens ALL the time! Not just to one or two individual files... but ALL the time.
Secondly I always download off big sites, like EA for patches for games etc and they never work!
Thirdly, sometimes friends refer me to a certain mirrors which they've used to download it from but when I use it, the file ends up being corrupted while theirs seem perfectly fine.

I don't know what's the problem but I hope you guys can help me out a little here.

I am using a 56k dail up with proxy addresses and ports.
thanks ppl!