I dont know if this will work:
Go to the Modem Settings and look for an option that says "Run Command" or somethin like that, and then there type "X0" (without the "), then restart the machine...
What it does is that it puts your modem to run at 128Kbps instead of 56Kbps, inproving bandwith by a little, but supported by some ISP, not all of them.
I've tried with 3 ISP, and just one worked out, the other ones didnt do nothing. So try that and see if it works with your modem and your ISP.