You can't. 12K is fast for a dial up. I used to get DL speeds of about 5K when I had dial up.
It's not a matter of machine speed or HD speed or Memory speed. It's the limit of the 56K access you have.
There are accelerators for netzero etc. But they just help loading pages I think. There is one thing that you can do. Use something that will split up the file into multiple streams that you can DL at the same time. It will help a little. Dial up is just dial up and that's it. Sorry.