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Here's how I went fixing my 3.3GB of Fedora i386/32-bit DVD ISO, downloaded directly(http) from an official mirror, which went kaput. Hash check(SHA1) failed to verify the ISO! Rather than a fresh download, I went fixing the ISO itself.

1. Download your data(ISO, ZIP, etc.) from any source or mirror. Sometimes it's faster to grab than a torrent download, but you may corrupt your big downloads with frequent pauses or crashes, and the checksum fails to verify!
2. Download your favorite Bit Torrent client. Here are some recommendations: µTorrent for Windows and KTorrent for KDE.
3. Search for the exactly same data torrent, and open using the torrent client.
4. Monitor the location of your torrent downloads directory while you start the download. Stop the torrent as soon as the files are created there.
5. Now replace the file in the torrent downloads with the one you downloaded earlier. Make sure the name is intact as the torrent client made it. For example, Fedora-9-i386-DVD.iso.bc! is the format for incomplete BitComet torrent downloads.
6. Now you need to check the data integrity of your torrent. The option is "Check data integrity" in KTorrent, and "Force Re-Check" in uTorrent(µTorrent). It'll take a few seconds to verify your data for corrupt parts and enlist to itself for re-download of them. My check stopped at 99.9% for Fedora-9-i386-DVD.iso.
   
7. When the check is finished, proceed with starting the torrent now. In my case, 0.01% was downloaded again, fixing my ISO!
8. Yay!! Now help others out! And keep seeding.

In whole, you are able to download stuff faster than torrents(sometimes ) via http/ftp sources, and later fix up any errors that download may have using a torrent client! Good luck!

Special thanks to the Compiz Fusion guy Jigish Gohil, who directed me to the way to fix it up! And to Martin Schlander (aka. “cb400f”) for the guide and KTorrent screenie!