Can a bad download cause my spin retry cycle to suddenly fail?

Defrag will not work, HFS+ works differently than Windows NTFS
Macs have background processes that maintain the index/folder structure to reduce fragmentation.

Corrupt download files will not affect physical performance of the disk

If a computer is dropped while running, that will be a good probable cause as to why the disk is on the brink of dying.

Put an SSD in there, will make a world of a difference!

That is strange... but there's no known software glitch/bug that will cause something like what you've described.

I'd say, write off the disks and put in a new SSD. The speed gained will overshadow anything that a hard drive can do.

Also, this may be a bit of a long shot, but check your fonts, see if there are any missing/duplicate/corrupt inside Font Book.

Allow me to say that it is far too early to write off the hard drive. You should first try some basic diagnostics.

But to give any suggestions for recovering from this situation you need to tell us what version of Mac OS X you are running. If it is 10.7 or higher, you can try booting to recovery, and repairing the hard drive:

http://www.apple.com/osx/recovery/

Post back if you need more than Apple tells you on that page.

When you boot with command and r (two keys) you will see the screen that is on the link I provided.

1. Click Disk Utility, and then click Continue.
2. In the list at the left, select the item you want to repair. (Be sure to select an item that’s indented to the right in the list, not an item at the far left.)
3. Click First Aid.
4. If Disk Utility tells you the disk is about to fail, back it up and replace it. You can’t repair it.
5. Click Repair Disk.If Disk Utility reports that the disk appears to be OK or has been repaired, you’re done.
6. Quit DU and restart as usual.

You do not lose anything on your computer, This is a process of checking the start up hard drive and making necessary repairs.

If Safari is the browser where the trauma started, try resetting Safari.

In the menu bar. click Safari and select Reset Safari.

If the Safari reset or Font check doesn't help, the issue is either at the software (OS X) or hardware (hard drive) level. In the IT world it all comes down to how much time you want to spend dealing with/fixing the problem.

This is a shot in the dark, but create a new user account, see how the performance is.

The last thing in my bag of tricks is to re-install the MacOS.

Run a Time Machine backup to an external hard drive first.
Be sure that you have all of your licence keys and installers for any paid software.

Then boot up in recovery and connect to the internet

Erase the disk

Run the MacOS installer.

Otherwise, if performance is not better with a fresh Lion installation, get a new disk