It could be the start of hard disk problems leading to the need for a replacement. Linux can often mount a volume when Windows cannot and also provides the means to rescuing your data. You can download a Linux operating system and run it as a LiveCD to pull your files off to anexternal source - portable drive or memory sticks. Find an OS at http://www.pclinuxos.com.
If you have an XP CD, try going as far as the first Repair option, get to Command Prompt and type the DIR command in then press Enter. If the disk mounts, you may have a chance of starting into Safe Mode.