It looks as the original had a 160GB 2.5" hard-drive and running Vista. You'll have to physically check if it's a Sata or Ide drive.I suspect Sata. Different connections shown Here.
If the original drive is still running then use it as little as possible so you can get someone to clone it. If you have the original restore to factory settings discs or the burnt ones you can use these on a new drive. Check key is still visible underneath or have you made a note of it?
Use something like Doubledriver to back-up all non Windows drivers if you can.
Back to the drive you can install a bigger hard-drive (not in physical size) such as a 250GB so long as it's 2.5".