Hi,
Your notebook is listed as coming with a 320Gb Hard Drive, but considering there's no real difference in cost, you can just as easily install a 500Gb or750Gb HDD if you want a bit more space - the drives on the following links are examples that would be fine for your notebook.
The procedure to replace the Hard Drive starts on Page 57 of your Maintenance & Service Guide.
Once done, simply use your Recovery DVDs to reinstall the Operating System on the new drive - this procedure is described in the HP document Here.
If you don't have your Recovery Discs, you can order a replacement set using the link below.
If you have any problem with this link, order them directly from HP.
If you live in the US, contact HP Here.
If you are in another part of the world, start Here.
Best regards,
DP-K