Why would adding 2 new chips of memory cause Win XP Pro Not to Boot?

I have a Gateway Athlon 700 with 584MB of memory. 1 256MB SDRAM chip
with ECC, 2 128MB chips, non ECC. Got 2 256MB SpecTek Non ECC chips
from my brother from his old computer. Replaced the 2 128 chips with
the 2 256MB chips, figuring no problems would occur. Lo and behold,
boot the system up and get:

Windows could not start because of an error in the software.
Please report this problem as :
load needed DLLs for kernel
Please contact your support person to report this problem.

Take the 2 chips out, problem fixed, boots up fine. What about these
chips would make the system fail? They should be compatible with my
system.

Any help would be appreciated

thanks,
Oscar