I would say that a file has been corrupted on your drive. Trouble is, which file? The error handling on MS operating systems is pretty awful, but they are getting better - XP isn't bad at all. In the drive to make everything user friendly and easy to look at, most of the internal workings of the operating system have been successfully hidden. Which is great - millions of people are able to effectively use computers where they might otherwise have not been able to. The problem arises when something goes wrong and it is next to impossible to diagnose the problem...
When does the error occur? Is it when you are trying to run a specific program? You may be able to reinstall it and fix the problem.
Before doing anything though, try running Scandisk and Defrag - they can stop all manner of hard disk related evils.