hard drives are standard parts. if the old drive is still working use hdtune to read the drive info. (model number and size).
then see if it and ide or sata drive by the model info from the hard drive vendor. then you can look online like new egg for a replacement hard drive. it can be the same size or larger if you want. with the pc make sure there a restore cd or disk image to reinstall windows on the new hard drive. if there no disk you have to contact emachine or look on ebay for a set of disks.