Somehow your MBR (Master Boot Record) likely got corrupted or possibly your hard drive went bad coincidentally.
Simply, you will start your computer from your Windows CD/DVD install disk, then when at a DOS prompt ( D:\ ) you will then type: FDISK /MBR (press enter).
Find your Operating System install DVD and start your computer with it, then follow instructions within this discussion:
Restore Windows XP MBR without Windows XP disc
Alternatively, review my search results for other instructions:
fdisk mbr windows xp Google Search
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