its not designed for a speedy ATA 100 drive but if you have an old drive thats gathering dust, you might as well use it.
http://www.macmod.com/content/view/434/2/
The offender is labelled C12 and it's a 330uF 200V Electrolytic.
http://www.r3uk.co.uk/content/view/53/62/
You should follow antistatic precautions or you may cause damage.
http://www.r3uk.co.uk/content/view/46/61/
Open the battery cover and remove the battery.
http://www.r3uk.co.uk/content/view/98/61/
It is an unusual design though and great care must be taken in the reassembly or you will have a problem with thermal damage.
http://www.r3uk.co.uk/content/view/51/61/
The Revision A was the first iMac (released 1998) and used a CRT screen
http://www.r3uk.co.uk/content/view/47/61/
You either need to access the motherboard, storage drives or power supply. It's not necessary to disassemble everything.
http://www.r3uk.co.uk/content/view/43/61/
the PSU cooling fan is located at the TOP of the case
http://www.r3uk.co.uk/content/view/52/62/
This common fault is down to a blown surface mount fuse on the motherboard usually.
http://www.r3uk.co.uk/content/view/54/62/
You'll need a torx-8 and a small regular screwdriver as well as a very sharp knife.
http://homepage.mac.com/sysop/PhotoAlbum9.html