So I bought Myst IV after having read through these forums and decided that my DVD drive would probably be able to read the discs okay. I was quite disappointed when I started getting the notorious IS_STRING_NOT_DEFINED error during installation, on both discs. I worried that I might have to buy a new drive, since the error message does say that it has trouble copying certain files because of an I/O device error, until I found a simple solution in one little topic on these forums: turn off antivirus software. I shut down my Norton Antivirus, along with all the other background programs in my Windows quicklaunch bar, and installation and play worked perfectly. I just wanted to post this to call attention to the very possible effectiveness of that solution, because even though the error message would imply that your DVD drive can't read certain files on the discs, IS_STRING_NOT_DEFINED might have nothing at all to do with your drive. I worry people might be buying new drives when the solution could be very simple and a new drive might not even help. So if Myst's not working for you, try disabling your antivirus software, it's worth it!