Perhaps you were fooling around with Boxee. Or maybe you just found this USB thing lying in the gutter and you decided to bring it home to a warm, loving environment. Who knows? The problem is, there is this GPT partion-thingy keeping you from using all the space on your USB drive.
What you need is this handy (Windows) tool from HP. It formats anything with a USB plug (well, maybe not everything). Anyway, the GPT partition or whatever else is on there doesn’t stand a chance. Here’s the link:
Update: You can also reset a USB drive from the command line.