Assuming that the HD is functional,
and it usually is after a case like this, it is fairly straight forward to remove the HD and connect it to another computer.
Depending on the model of iMac, it sounds as though you have an iMac G5 there, it is very easy to remove the drive, only takes three screws to remove the back cover, and then recover the drive.
(while you have the back off, and just out of curiosity, check the capacitors for bulges and leaks. This model was notorious for that and may well have been the cause of the problem)
This model iMac used a SATA drive and, in my opinion, the easiest way to connect this drive to any other computer is with a SATA dock, like this one:
There are a number of different ones, I like the Thermaltake ones, some with eSata and Firewire but all with USB.
Once you have done your thing with the Mac disk, you can always format it for Windows and continue to use the SATA dock.
To read the drive on another Mac would not be a problem but to read it on a Windows machine you will need a piece of software that enables Windows to read the HFS+ formatted drive.
This one is free: http://www.catacombae.org/hfsx.html
while this one is not: http://www.paragon-software.com/home/hfs-windows/ but it is easy to use and works well
Hope this helps