Hi there,
Its really Simple to change your Hard drive.
Take note of this:

Do you have a SATA or ATA (the Pin configuration on the back)
A SATA will have two flat connectors (the short one is Power and the long one Data) they also have a few Jumper Pins to set as Master, Slave, Cable Select. A diagram will be on top of drive.

An ATA will have a load of pins (44).
Be sure to take any adapters of the hard drive, By adapter i mean any pin converter that is plugged into the back of a drive for example on a Toshiba there is an adapter that turns a ATA pins into a Slot connector.

Enougth about ATA and SATA...
Your drive will probably be 4200 rpm or 5200 rpm or 7200 rpm
Obviously the faster the better so i would sugest 7200 rpm if you can find one otherwise a 5200 rpm will do. So the speed wont be a compatible issue. If anyone needs help on replacing a Hard drive or opening a laptop then a Video DVD can be found at Home
hope this helps
Carl