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I've been having issues with the Windows 98 SE OEM Full ISO. It boots in VirtualBox, but if I select options 1 or 2, it can't access the CD-ROM drive meaning I can't install it. Is there something wrong with the file itself?
Shouldn't be anything wrong with it. Make sure VirtualBox is set to emulate an IDE CD drive. Also perhaps try changing the IDE channel the emulated CD is connected to.
While some people like booting from CDs, the most reliable way to set up Windows 9x is to use a 9x boot floppy, partition and format the drive, then copy the setup folder to a folder on the hard drive, then run setup from there.