[QUOTE=drewbert]
OS 7.5.3 is available as a free download from Apple - IMHO they should make all versions of the old OS free now. Later versions are probably floating around on eBay for not much.
[/QUOTE]
7.5.3 won’t run on any iMac.
Without knowing what model iMac it is, it’s not possible to determine the maximum supported OS. Your iMac definitely supports up to OS 9, and at least up to OS X 10.3. Best bet is probably OS 9, should you find a copy. OS X is likely to be slow, especially if you have less than 512 MB RAM (it is almost certain that the machine does not currently have that much, it’s probably in the 32-128 MB range).
Repairing the current install is going to be a bit difficult if you don’t have a lot of Mac troubleshooting experience. You can get pretty far, however, by deleting all unwanted apps from the hard drive, and going into the System Folder and deleting the contents of the Preferences folder.
If you still have problems, open Extensions Manager (Apple Menu -> Control Panel) and choosing the “Mac OS 8.6 Base” or “Mac OS 8.6 All” option from the “Extension Set” menu. Reboot when prompted. That will disable all non-Apple extensions. From there, you can selectively re-enable extensions as needed.
Hold down the shift key when booting to disable extensions (like “safe mode”).
Extensions are the Mac analogue to drivers on Windows.
Disk Utility may not be able to fix the disk, since you are booting from the disk that needs repair.
The iCab browser is the most up-to-date browser for pre-OS X systems.
Configure network access by going to the Apple menu, then Control Panel, then TCP/IP. Make sure “Connect via” is set to Ethernet and “Use 802.3” is unchecked. Change the other settings as needed to work on your network. There’s no OK button, just close the window when you are done. You may have to answer OK to a prompt to activate TCP/IP.
As far as identifying the exact model, does the iMac have any of the following?
Slot-load CD drive
Tray-load CD drive
Firewire ports
Transparent top case
A little plastic door covering some of the ports
Also, check Apple System Profiler (usually in the Apple menu) for detailed specifications. Knowing the processor speed will be very helpful in identifying the model.