Can’t send packet bigger then 1472 bit

I’m connected to the internet via ADSL.
I can’t send packet bigger then 1472b (wich means that I can’t send any file or email biger then this size), but files can be downloaded from my computer. I tried to change MTU value in the registery to 1024 but the problem remains.
How can I change the packet size in Windows 2000? or What else I can do so it will work?