Adding DDR Memory

I recently tried to add an additional stick of 256/266
DDR Ram to my system but when I do XP won't load. The
start-up keeps restarting.

Even when I try to start in Safe Mode.

I got once to an error message saying there was an error
detected in my windows/sys32 file which I assume is the
startup info which previously would have shown 256 Ram
and not 512 with the new DDR Ram.

Anyone else have any problems and if so how dod you fix.

Thanks