The hard drive in my Early 2008 iMac 20" (EMC 2210) was throwing all kinds of S.M.A.R.T. errors about and was, in general, acting very strange. Ultimately, I knew what had to be done: follow the iFixit Repair guide, gather the necessary parts / tools and set to work.
All-in-all, the repair took about an hour... and that's because I was going SUPER slow as to be meticulous: using Styrofoam coffee cups (labeled with a Sharpie as to what type of screws they were and which step from the guide they came from) to hold screws as I removed them. I followed the guide to the dot... and it was super helpful.
The only thing that threw me was Step 1... I purchased the recommended Phillips #1 screwdriver, but it definitely did not fit the Phillips screw on the access door. Had to bust out the Phillips bit on my Leatherman to loosen the Phillips screw. Not a horrible thing... I now have a handy-dandy Phillips #1 screwdriver... just not sure why it didn't fit the screw it was outlined for? Perhaps my access door is weird... thanks Apple!
Also, the repair guide (in Step 14) suggests that releasing the hard drive bracket "requires a substantial amount of force". Again, perhaps my iMac is special... but it did not require a ton of force. Just saying... take it easy and hopefully you won't Hulk-smash your iMac in the process.
TAKE EXTREME CARE in handling the glass panel and display panel. The repair guide was not kidding that any kind of dust / finger prints / crud you don't remove will show up in-between the glass panel and display panel once everything is back together. I used a lint-free cloth and a can of air to make sure everything was neat and clean before reassembly. While performing the repair, make EXTRA sure you don't touch the under-side of the glass panel or the front of the display panel... this will make for less cleaning and a speedy reassembly.
Phillips #1 Screwdriver
Heavy-Duty Suction Cups (Pair)
TR8 Torx Security Screwdriver
TR6 Torx Security Screwdriver