For Windows 98,98SE:
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You can go to Start --> Find --> Files or folders and look for base4.cab file on the C:\ drive.
If you find it in a folder like C:\Windows\Options\Cabs or C:\Windows\Options\Install then that
would be the folder from where you can reinstall Windows using a setup.exe file.
For Windows ME:
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Go to Start --> Search for files or folders --> look for base2.cab file on the C:\ drive.
Again the folder where it finds the file is going to be the folder from where you can reinstall windows.