If this is a Old Laptop, it may have a BIOS limitation on size. PS My Laptop is slow it's nothing to do with Memory, I fitted another 256mb it does not affect the speed unless you are doing Photo Editing etc. The problem is the speed of your Hard drive, Only 4200rpm on the old one. That's what mine is but going to get this.
Hitachi 7200rpm high speed Laptop Hard Drives
Hitachi E7K60 and 7K100 2.5" hard drive series are the first two 7200rpm high speed laptop hard drive in the market.
60GB Hitachi HTS721060G9AT00 £96.98 (inc VAT) in stock
From click here