iMac started to take 10-15 minutes to boot and launching apps like Skype started taking very long time.
The usual suspects were memory and hard drive. Replacing memory with two 4GB sticks didn't help. I decided to replace a HDD with a spare one I had.
Following the instructions was super easy, especially with the Pro Tech Toolkit I bought a week earlier (Thanks for quick shipping by the way - I'm in Australia and got the kit 3 days after I paid for it!).
Right before I was about to remove the hard drive I saw the CMOS battery and thought it might be a good idea to replace it too as it was now almost 4 years old.
Done that, replaced the HDD and installed the latest version of Mac OS. Everything works like a charm now!
1) When you disconnect temperature sensors and the mic cable, remember which way they go. They are small and plastic, its easy to put them in the wrong way.
2) The Mic connector is pretty flimsy. Unwrap it very carefully. When assembling the iMac, it pays off to boot the machine before you put the protective panel back to make sure the mic is connected properly. I didn't do that and had to take the panel off again.
3) I didn't have suction cups and didn't want to buy them just for this one-off job. So I used a suction mount from my car GPS and an old Atari joystick (it has four little suction cups underneath). Worked pretty well!