UPGRADING MY HARD DRIVE

Hi,
My compaq pc is 5years old and i would like to replace the hard drive, presently the capacity is just 40gb which i feel is insufficient because i do lot of video projects. I want to go for a higher one.
My o/s is win xp home sp3 updated. I have been provided with recovery discs by compaq. What are the procedures i have to follow to replace the current hard drive and identifying the type of hard drive i need. Please help me in this regard.
Thanks…
mcsujit