You need to access the hard drive. First, open a terminal window (right click on the desktop and select open terminal). Type mkdir /mnt/hda1. Type mount /dev/hda1 /mnt/hda1. A few things to mention about this. First, mount may fail saying that you must specify a filesystem type. Chances are, the hard drive has been formatted with either NTFS or FAT32, both of which can be determined automatically by mount. If mount cannot determine this, then the drive is most likely trashed or in need of a reformat. A second thing about filesystems. If the hard drive was formatted with NTFS, the default for newer versions of Windows, then you will not be able to write to the hard drive by default. You can change this, but it is not recommended (believe me-I tried that and corrupted the entire drive with a single write of 1MB).
Assuming all went well...
You now need to actually view the files. This can be done graphically with a program called file browser. It is located somewhere under the applications menu (I forget exactly where). Go up a few directories until you see the mnt folder. Double click that, then double click hda1. You will see the contents of the entire hard drive (hopefully). You can right click a file to copy it and so on. Just remember that anything you copy, if not being placed on another hard drive or over a network, will actually be placed in RAM. RAM is limited, so don't try this with anything too big.