My Device Manager says mine is Generic IDE Disk Type 40. How can I tell if it's Western Digital?

I found this on Microsoft's support. Apparently a Western Digital drive can cause problems if used as a slave in an XP system.

http://support.microsoft.com/default.aspx?scid=kb;en-us;812581

SYMPTOMS
When you try to configure a secondary hard disk on your computer to be a subordinate (also known as slave) drive, you may receive the following error message:
There was a problem installing your hardware,
Code 28 one of the Filter Drivers installed for this device is invalid.

CAUSE
This issue occurs because filter drivers for certain types of hard disks are not valid for other hard disks. This issue is most common when you install a Western Digital hard disk as a subordinate drive in a computer with a different manufacturer's hard disk is installed as the master drive.

Reading down the page you quoted it does say that it only applies to XP Professional Edition. The problem occurs when you install the hard drive, so if you've passed that hurdle it should continue to work correctly. If you really want to know the manufacturer it probably means taking the cover off your computer and seeing if you can read the label on the drive.

The Belarc Advisor is a good utility for displaying all sorts of information about what's inside a computer.

http://www.belarc.com/free_download.html