Found this in the Microsoft.com knowledge base....
"Logical Assertion Has Failed" Error Message When You Install Windows (Q292763)
--------------------------------------------------------------------------------
The information in this article applies to:
Microsoft Windows 95
Microsoft Windows 98
Microsoft Windows Millennium Edition
--------------------------------------------------------------------------------
If this article does not describe your hardware-related issue, please see the following Microsoft Web site to view more articles about hardware:
http://support.microsoft.com/support/windows/topics/hardware/hwddresctr.asp
SYMPTOMS
When you attempt to install Microsoft Windows 95, Microsoft Windows 98, or Microsoft Windows Millennium Edition (Me) on your computer, you may receive an error message similar to one of the following when Microsoft ScanDisk checks the computer drive before Setup starts:
Logical Assertion has Failed
Expression: IDXStart
Source: line 264 of msprintf:c
-or-
Logical Assertion Failed
Expression IDXStart <= nMem Blocks
Source: line 266
File msprintf.c
After you receive one of these error messages, you are unable to install Windows on the computer.
NOTE : You may also receive one of these error messages during Setup.
CAUSE
This behavior can occur because of a defective or incorrectly configured hard disk or related hardware component.
This behavior can also occur because of corrupted hard-disk drive overlay software.
RESOLUTION
To resolve this issue, quit Windows Setup and use the following check list to troubleshoot the source of the error message:
Check the physical connections to the hard disk. For example, check that all cables are properly oriented and connected.
Check all jumper settings on the hard disk.
Replace the ribbon cable on the hard disk.
Check the Basic Input Output System (BIOS) settings for the hard disk to verify the proper configuration.
NOTE : For information about how to gain access to the computer BIOS, consult your computer's owners manual.
Test the hard disk controller and/or the hard disk itself. To do so, either use third-party testing software or temporarily replace the computer's motherboard.
NOTE : The error message may be caused by a defective Integrated Drive Electronics (IDE) hard disk.