Hard drive is important in all computers as it is the place where you will be storing all the data. The most common hard drive is IDE drive and installing this is a bit tricky as you have to use jumpers. If the drive is Serial ATA drive, then there will be no jumpers and installation is much easier.
The motherboard will generally have two IDE connectors. You have to use IDE2 for connecting DVD/CD drivers and IDE1 for connecting hard drive. Remove the computer case and locate the socket where the hard drive has to be installed. If you are installing a new drive, then jumper settings are important. The jumpers are metal pins that are used for configuration of hard drives. You will have the jumper settings in the hard drive manual.
For single drive, you have to use master jumper selector. For two drives, if you want to install mail drive, set the jumper to master otherwise set it to slave. Then gently push the drive into the drive bay and line up the holes for screwing the drive. One end of the IDE cable has to be connected to the motherboard and the other end to the hard drive depending on master or slave. Similarly, power cables should also be connected to the hard drive to supply power. For SATA hard drives, jumper settings are not needed and the rest is the same.