RAM in my Dell CPx

Help!!!!
I have a dell cpx with 128mb ram......30gb hard disc. I have bought a SODIMM 256mb memory stick, which is compatable with the SODIMM 128mb existing memory.
The screen displays that the memory has been changed when I put the memory into the slot, but windows cannot start. When I take the memory out, screen displays that memory has been changed and windows starts as normal.
Any ideas please.
PS....in case you hadnt guessed....I am in no way an expert!!!!

Jack

 

Are you replacing the 128 stick with the 256 stick or adding to the 128 stick to give you 384mb ram?
It could be possible that your ram is not compatible with your motherboard, i.e. that your 129mb stick is say pc2700, the max. that your motherboard will support and the new 256mb stick is pc3200 - therefore not being compatible with your motherboard. The more detail you supply about your memory, the more we can help