Command.com is the backbone to DOS. It is the program that gives you your command prompt, and also may be used to do file transfers.
If you have one (you should have), put a boot disk into the floppy drive when this error happens and in the box type "A:\COMMAND.COM" without the speech marks. Click ok and see if that helps.
You may need to remove the disk after the computer has finished with it.
This idea comes from the fact that Command.com it deleted or moved or renamed during the setup process, when Setup obviously has not finished with it. Since the command prompt is not visible, Setup catches the error and throws up a box. If anyone else from the Forum has a fix or workaround, please help out as this is the only advice I can give.