Force Compatibility Mode Disk Access

I am troubleshooting a friend's Win98 computer.  An attempt was made in install an M-Systems Disk on Key [] device without the Win98 driver installed first.  Now the system gives the message: "Windows Protection Error.  You need to restart your computer" upon bootup.

I have narrowed it down to this:

When I run msconfig, and select "Force Compatibility Mode Disk Access" under the advanced tab, the computer will start.

From Microsoft's website:
Force Compatibility Mode Disk Access
As you know, Windows 98 uses 32-bit drivers for all of the disks in the system. If, when you check the Performance tab in the System Properties dialog box, you discover that one or more of the hard disks in your system is using MS-DOS Compatibility mode due to disk peripheral input/output (I/O) problems, you can use the Force Compatibility Mode Disk Access setting to troubleshoot the problem. When you enable this setting, it prevents Windows 98 from using 32-bit disk drivers, except for the floppy driver, and forces it to use 16-bit, or real-mode, drivers or the BIOS to manage the drives. Keep in mind that any disk that only works in protected mode will no longer function. Using this setting is also equivalent to starting Windows 98 with the /d:f command-line switch.

My question is HOW do I troubleshoot it?  What do I check, and what can I change to resolve the problem?
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has the driver been installed since then?
sssobelAuthor Commented:
Yes it has.
roll back the registry to date before M-Systems Disk  was installed.
boot to dos  and type:  scanreg/ restore
pick a date before the install process and reboot the machine.
Now load the win 98 driver then the M-Systems Disk

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sssobelAuthor Commented:
That solved the problem!  Perfect, thanks!
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