Hi,
Your notebook is listed as coming with a 250Gb HDD, but you may as well replace this with a 500Gb drive as there's no real difference in cost.
The drive on the link below is an example of one that would be fine for your notebook.
The procedure to replace the Hard Drive starts on Page 54 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.
Regards,
DP-K