for replacing the hard drive, you will either need to attempt a full backup of a drive (google "backup hard drive __state problem__") or start fresh. There are different types of hard drive connectors (IDE, SATA, SATA II, SATA III, etc.) you must make sure you purchase the correct drive when you do so.
if you have an IDE 2.5in laptop drive with 8gb capacity, you could go online and find another IDE 2.5in laptop drive with a 40gb capacity and when you plug it in it will be blank. you can transfer your files over, but it is unwise to transfer the windows installation (if possible) so you will need a windows install disk. these can be downloaded from many places around the internet, and will work with the retail windows activation sticker located on the bottom of your laptop, or the back or side of your desktop.
this is just an idea to point you in the right direction, if you use a search engine, you will most likely find an answer to your problem