This is a serious problem that has been plaguing me for months.

Whenever I try to download or install (from a CD) large files, they usually become corrupted, namely the larger parts of the files (MPQ files in Blizzard games, or large level files for instance).
This prevents me from properly running/running a game.
Is there anyway to stop this? Do any of you know the source of the problem?

Dicky CD drive? Have you got another one around to try?

Is the disk old, scratched or showing signs of age?

No, first of all, it isn't the CD-Drive, because I already said in my post that it affects both downloads, and installation via the drive.

Secondly, it's nothing to do with the disks themselves, a brand new CD of Half Life 2 refuses to install correctly without corruption, yet it works perfectly on my other PC.

I really don't know the problem. Could my hardrive be corrupt perhaps?

oops, missed that bit about downloads.
Sounds like a problem with the HDD then. Try a run of scandisk.

Of course it's your HD giving it's spirit up. The only way is to get a new HD.

Yes, if you are sure it's not the CD or the internet, then it must be your HD. I completely agree with those two above me.

If the HDD is on its way out (which seems likely to me, though I'm no expert by any means) it's best to replace it ASAP and copy the files you want over (in decreasing order of how much you want to keep them) - when they fail, it can be quite sudden and near-impossible to recover anything.

Having replaced it, I'd try leaving both drives in and formatting/scanning the old one - it might be recoverable, in which case you've got an extra hard drive out of it.

It's been through multiple re-formattings, so I do think it's getting a bit past it's prime.

Can also be RAM. I've experienced it myself, and all was caused by a faulty pair of DIMMs.