Do you hear a clicking sound when you are running your computer? When you are writing data to the drive, does it get worse? If so, try running the hard drives diagnostic. You can get it from the hard drives website. The software will tell you if the drive is headed for the grave or not.
Have you tried running checkdisk with the /r /f flags? Go to run and type chkdsk /r /f and hit enter, say Y to the question and reboot the machine. Go watch a movie...it takes that long! Do Not stop the process when you have it start or you will bork windows. Now see if your downloads improved.
My XP machine sees 3.3 gigs of ram, I bought 4 gigs because it was so cheeeeap and it gives me dual channel to boot!