OK here is what to do...
Download DriveimageXML (
DriveImage XML - Free software downloads and reviews - CNET Download.com) on your current system.
Shutdown and plug in your replacement drive so both drives are connected.
Boot the computer off the original disk and select to run drive image.
Within drive image select the clone option, its best to clone physical disks rather than logical partitions as it will grab all partitons off of the source disk regardless of whether they are active or not.
Select your new disk as the destination (You do have the option of an image file but you dont need this).
Then leave it running.. it can take some time dependent upon disk size - I think my 120gb disk took around half an hour.
After its complete you may now take the old disk out and the system should boot off your new disk with everything looking exactly how it was.
If you are feeling brave you could in the bios change which disk is boot disk and reformat your old disk but if its on its way out its best not to store anything of use on it. Personally I put mine in a draw should I ever need to go back and re-image that disk.
If your still feeling unsure there are utube videos showing this being run which can be seen here
YouTube - Runtime Software DriveImage XML tutorial
Cheers
Jof