Bad Master Boot Record (MBR). Now you have to determine what caused it and more importantly determine if it will happen again. When you originally installed XP did you do an upgrade or clean installation? A bad installation will corrupt the MBR. This happens when you are formatting the hard drive and you receive an error. It is even possible to have more than one MBR and when XP is loading it reads the wrong MBR. It is not rare to run XP for months and run into the corrupt MBR issue.

The short, but best fix is to always use the manufacturer's hard disk utility to configure a hard drive before installing XP.