The main two things are the speed of your broadband and the speed of the machine you are downloading it from.
It is very little to do with the speed of your computer.
You can up the speed of your broadband by paying for a faster broadband.
If you are downloading from a file sharing system or torrent (so getting data from someone elses PC) you cannot do much about that.
You can try using the internet during the night or early in the morning.
I find everything goes a bit faster when I try to download things during the night or early in the morning, less overall network traffice I suppose.