If your computer is using the older MBR boot mechanism your computer would be booting from the boot volume which is named System Reserved, you can see this by clicking on the Start button > right click Computer and selecting Manage then select Disk Management. This partition should have the System and Active flags set. The second partition will be your C:\ Drive or system volume. It should haveat least the Boot flag set. Please upload a screen shot so we can verify this.
Next open an elevated command prompt (press the Start Button, type cmd, right click Command Prompt and select Run As Administrator)
Type bcdedit /enum and also copy those results here for review.
It's also possible that when you install Windows XP it...