Hi, if you are downloading these files from the internet the most likely cause is a slow Internet provider.
Goggle 'internet speed test' and the name of your ISP (verizon, your cable company, whatever your use to attach to the internet). Run the speed test and compare to what you are paying for. Then complain if you are not getting what you are paying for.
A likely cause is due to your computer running background malware/virus checks. If you are running something along the lines of Norton or Symantec as an antivirus, your cpu and drive susbsystem may simply be bogged down running the checks.
Open your task manager, select "processes" tab and click on the "CPU" bar to order your list into those processes using the most cpu cycles. It is a decent way of telling you what the most "cycle consuming" process is. Also look to see if your harddrive light is constantly flashing.
Generally, if your PC has slowed to a crawl, it is because your PC has contracted a virus or it is simply checking for viruses. This problem is more prevalent on older PCs due to the lack of processor power trying to run bloated antivirus software.