This is still not making sence

Crucial is top of the line. I ran your board thru other scans & found the same memory as compatible.

Hmmmmmmm...

K7S5A V3.X

512MB 184-pin DIMM CT6464Z265 DDR PC2100 • CL=2.5 • UNBUFFERED • NON-ECC • DDR266 • 2.5V • 64Meg x 64

MEMORY

2 DDR DIMM sockets and 2 DIMM sockets support (can not be used simultaneously)
Two 184-pin DDR SDRAM (DDR200, DDR266) or
Two 168-pin 3.3V SDRAM (PC100, PC133)
Maximum: 1.0GB (DDR or SDRAM)

Memory · Two 168-pin DIMM slots for SDRAM memory modules
· Two 184-pin DIMM slots for DDR memory modules
· Support SDRAM up to 133 MHz /DDR up to 266 MHz
memory bus
· Maximum installed memory is 1GB
Note: You cannot use SDRAM and DDR simultaneously.

DDR SDRAM provides 800 MBps or 1 GBps data transfer depending on
whether the bus is 100 MHz or 133 MHz. It doubles the rate to 1.6 GBps and
2.1 GBps by transferring data on both the rising and falling edges of the clock.
DDR SDRAM uses additional power and ground lines and requires 184-pin
2.5V unbuffered DIMM module rather than the 168-pin 3.3V unbuffered
DIMMs used by SDRAM.

System Type: Motherboards
Manufacturer: ECS Elitegroup Computer
Model Name: K7S5A V3.X
Bus Type:
HD Bus Type:
CPU: AMD Athlon or AMD Athlon XP or AMD Duron
Standard Memory: 0 MB (removable)
Max Memory: 1.0 GB
Number of Sockets: 2 sockets DDR / 2 sockets SDRAM
Native OS:
Memory
DMS # Capacity Description Model Price Buy
DM50 602 128MB Module 128MB DDR PC2100 266MHz (Non ECC) 16x64 CL2.5 2.5v 184 Pin DIMM K7S5A V3.X $19.00
DM50 573-1 128MB Module 128MB PC133 168 Pin CL3 16X64-7 Unregistered SDRAM (4 CHIP) K7S5A V3.X $21.00
DM50 553-1 128MB Module 128MB 168 Pin PC100 16X64-8 CL2 Unregisterd SDRAM DIMM (4 Chip) K7S5A V3.X $21.00
DM50 603 256MB Module 256MB DDR PC2100 266MHz (Non ECC) 32x64 CL2.5 2.5v 184 Pin DIMM K7S5A V3.X $27.00
DM50 574-1 256MB Module 256MB PC133 168 Pin CL3 32X64-7 Unregistered SDRAM DIMM (8 Chip) K7S5A V3.X $35.00
DM50 554-1 256MB Module 256MB 168 PIN PC100 32X64-8 CL2 Unregistered SDRAM DIMM (8 CHIP) K7S5A V3.X $35.00
DM50 604 512MB Module 512MB DDR PC2100 266MHz (Non ECC) 64x64 CL2.5 2.5v 184 Pin DIMM K7S5A V3.X $49.00
DM50 575 512MB Module 512MB PC133 168 Pin CL3 64X64-7 Unregistered SDRAM DIMM (16 Chip 32x8 Based) K7S5A V3.X $83.00
DM50 556 512MB Module 512MB 168 Pin PC100 64X64-8 CL2 Unregisterd SDRAM DIMM (16 Chip) K7S5A V3.X $83.00
DM50 605 1GB Module 1GB DDR PC2100 266MHz (Non ECC) 64x64 CL2.5 2.5v 184 Pin DIMM K7S5A V3.X $109.00

I wonder if Clearing the CMOS could help? Don't see how it would hurt.

JP4: Clear CMOS Jumper
Use this jumper to clear the contents of the CMOS memory. You may need to
clear the CMOS memory if the settings in the Setup Utility are incorrect and
prevent your mainboard from operating. To clear the CMOS memory, disconnect all the power cables from the mainboard and then move the jumper cap into the CLEAR setting for a few seconds.

Although it is not mentioned in your manual most often it is advised to also remove the battery (disk below jumpers) during this process.



If still no go there is this setting in your BIOS:

Auto Detect DIMM/PCI Clk
Use this item to enable the DIMMs or PCI slots to detect automatically device
then generating clock.

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Just an added thought that maybe someone else will have an idea about as I see nothing in your manual about making changes here but:

The 184-pin requires 2.5V

Whereas:

The 168-pin uses 3.3V

I would assume that this is handled on the board but am not sure


_____________________________________

hi!

you can check out this site. i have same problem as yours only that im using pc3200.

Unofficial K7S5A Guide

HTH.