The difference between a 40GB and a 60GB is a 60GB has 20GB more capacity then the 40GB, but it's going to be a fresh harddrive, so you will need to install a new OS (Operating System, meaning "windows 2000, XP, etc.."), unless compaq has it installed already, and you will lose all previous information on your 40GB, because you replaced it with a 60GB. So I would recommend burn/backing up all important information onto a CD. (If possible)

Or, if you where talking about adding an additional harddrive (which I didn't know you could do with a laptop), the 40GB would be your master and the 60GB would be your slave, so you would have 60GB extra.

later,
Dru