In addition to what Stephanie has just said may I ask, is the speed you are receiving the speed that was advertised by your ISP? It is possible for your ISP to cap your download speed based on the package and price you are paying.
Also you say your speed varies from
80Kbps - 1000Kbps (1Mbps)
This is a huge range if what you are saying is correct... Do you mean 800Kbps? If not then I would strongly consider ringing your ISP to find the problem since your speed should not vary that much. As for your initial question, there is very little you can do on your machine to directly improve your download speed and any settings that would affect it will be on your router.
Hope this helps,