There are a few things that can cause this, and one is very easily correctable:
1. a removable drive was plugged in and the bios is trying to boot from it instead of the main hard drive. - In this case, you can go into the bios and set your boot order to exclude the removable drive or just remember to unplug the drive when you boot up.
2. software corruption damaged the boot files. - In this case, you can possibly correct it by booting up from the Windows XP CD and access the repair console. From there, you can use the "fdisk /mbr" command to fix the boot sector or run the procedure listed here:
http://pcsupport.about.com/od/fixtheproblem/ht/ntldrntdetect.htm Installing Windows 7 would fix it, too, if you want to do so.
3. damaged hard drive. - If this is the case, trying to install any OS would not help. you will have to replace your hard drive first. To find out if you have a bad drive, boot from the Windows XP CD and run "chkdsk c: /r" and let it run. It will come back with either a fix or a massage that it can't be fixed.