Most of the time I download compressed files I received CRC errors. What software is out there that can check if my hard drives have gone bad? Also, could there be any other factors, besides bad hard drives?

Could be the compression program. Some compression programs have problems with different types of compression and they error out with CRC error's. Try using Winrar/Powerarchiver/Winzip and see if the errors still occur.

Also what Inet connection are you using to connect?
Do the downloads finish downloading?

I'm using Bell Sympatico High Speed Edition (ADSL) and all the downloads finish, with the expected file sizes.

A friend of mine had problems with large downloads being corrupted; turned out to be his ethernet card going bad.

yea if the extraction program you are using is too old then it will give you errors reading newer file formats.

Is there any way I can test my ethernet card to see if it has gone bad?

If you've got another computer networked to yours, try moving known good compressed files back and forth. Then extract and see if they still work.

I copied a known good compressed file to a networked computer, confirmed a good CRC there, and then copied it back again to my computer. The file kept getting CRC errors on my computer. When I went to the other computer and checked the CRC of the file it was good every time. I'm now pretty sure my NIC is to blame, but have I really narrowed it down to that?

Did you check your BIOS-Settings? I don't know anymore which setting it is but it has to do with RAM... I can switch between 1T and 2T... 1T is faster so I tried it... The result was that almost every RAR-Archiv I extracted had CRC-Errors... I tried to extract it once again and it worked... So it wasn't really corrupt - my comp made a mistake cause of the RAM-Seetings... I switched back to 2T and everything is back to normal... Maybe you too have activated "1T" ...

ECO, do you have broadband? Are you running a router? I've seen routers corrupt downloads, which can be fixed by a firmware flash... Check your router manufacturer's web site, if applicable. Let us know what you find out, ok?

A few more things to try, run a port scanner on your machine, look for anything "odd".. Also, are you running any CURRENT antivirus software? Just some more tips I had thought of. Good luck.