Gary is right in what he says, but also cheap memory can be problematic, even if it is the right type. Also, some memory sticks won't work together. Have you installed it with your existing memory? Try using the new one only. Also, try swapping the slots the memory is in. I once bought a 256 stick, and it wouldn't work. Took my existing stick out, replaced it with the new memory and it worked. Put the old memory in the spare slot, and hey presto! No problems.
Paul Higgins, July 2005