Considering the fact that model iMac is too old to use Internet Recovery, explain how you arrived at that step — installing OS X El Capitan. That background information may help us understand what's going on.
The dialog alleging corruption is overly simplistic but effectively correct. It can't be used.
Having had system corruption followed by installing Microsoft Windows on that Mac, the eventual fix will almost certainly require booting from that Mac's original, grey, System Install DVD. It will offer to install its shipping version of OS X, whatever that may have been. You will almost certainly need to reformat its startup disk as a prerequisite. Then, you can upgrade it to El Cap using the instructions in How to download and install macOS - Apple Support. Scroll down to El Capitan 10.11 to download the .dmg file.
I have no idea what will happen if you download that file using a Windows PC. I suspect it won't work.
Since that Mac's original OS X version of Safari probably won't work at all, nor will its version of Mail, nor do you have another Mac with which it can be networked, copy the El Cap .dmg file to a flash drive or burn it to CD or otherwise find a way to deposit the file onto that Mac's startup drive. Open the .dmg file and proceed from there. As I alluded to earlier you probably need to download that .dmg file using an operable Mac.
