An harddrive CAN affect your download speed,
if
1. that HDD is dying
2. if your using too much programs doing I/Os on your drive ( ie. running a virus check, a defrag, a bittorrent downloader, and god knows what else, at the same type)
thought your hdd has to be shitty if its that under 1MB/s, but with realy high speeds over 800KB/s it could be it
3. Having low ram, low initial pagefile, and using shitty windows management of pagefile.
4. a combination of the 3
5. some other stuff i didnt think about on the top of my head
in all those case the download speed is slowed down because of the I/O limit of the HDD
i myself have a dying hdd and i must asdmit that while it slwoed down my games loading time considerably, it havent slowed down my downlaod speed t5o much yet..