You just need to redownload the bittorrent client. Then click the link to the torrent you were downloading and save it to the same place as you did before it'll pick up where it left off.

What speed do you upload at?

Let me give you a quick rundown on how tcp/ip works...
tcp/ip requires a certain amount of overhead to transfer the packets back and forth.
Everytime tcp recieves a packet it sends out another packet telling the 'server' that it recieved it and then the 'server' sends another packet. If you max out your upload (speed at which you send out information) then your download (speed at which you recieve information) will decrease. This will cause your download to be about the same as your upload. Best idea is to limit your upload to about 6-10kb under your max upload, which in your case since you are on cable, is about 35-50kb/sec. Both of the clients I posted above allow you to limit your upload.