Yeah, it is kinda bad marketing, but in most cases they give the actual size of the hard drive under Windows XP somewhere in tiny print on the box. If it's OEM, then chances are it doesn't have it anywhere...
Once terabytes become standard, you'll only be getting like 91 percent, so it's gonna get crazy after a few generations of hard drives. And just to clarify, nothing at all is wrong with your hard drive.
Sorry this situation happened. 372GB is still a ton of space though
. Way more than I need, anyway.