If it is the drive that contains your operating system, then you will need to either:
1) make an image of your current drive with an appropriate tool like acronis and restore to your new drive.
(http://www.acronis.com/homecomputing/products/trueimage/)
or
2) re-install your operating system on the new drive and all your other software.
If you are replacing a data drive then you will need to backup the contents and restore to the new drive
Points to watch:
a) check the 'jumper' connectors on the drive you are
replacing that determine if it is a master or salve otr cable select drive. Ensure the new drive is set the same.
See this .. http://www.harddriveupgrade.com/
and good luck
Trev
May 2007