I just bought 2 WD 500GB hard drives and installed one in each of my computers. They were detected at boot time but not showing in My Computer as an available drive. Here's how you get XP to detect new drives if it doesn't automatically do so:

NOTE: This procedure is for adding a new, blank hard drive to your computer only.

Occasionally, a hard drive may not be detected by a computer running Windows XP. If that is the case, you may need to initialize (write a signature to) the drive. If your drive is not detected by Windows XP when you check in Disk Management, follow the steps below.

Note: In some cases, the Disk Signature Wizard will start immediately when you run the Disk Management utility for the first time after adding a new drive.

1. Right-click the My Computer icon on the desktop. If there is not a My Computer icon on the desktop, you can find the My Computer icon on the Start menu.
2. Left-click on Manage.
3. Left-click on Disk Management.
4. The Initialize and Convert Disk Wizard will appear.
5. Left-click on the Next button.
6. Check the box to the left of the drive that you want to write a signature to.
7. Left-click the Next button.
8. Left-click the Finish button.

The drive is now initialized. You should now be able to create a partition and format it.

WESTERN DIGITAL Caviar SE16 Hard Drive
MFG Part #: WD5000AAKS