In most cases this is caused by your computer trying to boot from non-bootable devices.
Remove any floppy disks, CDs, DVDs you might have in your floppy, CD, and DVD drives. Do you have your digital camera or mp3 player plugged in? Unplug them as well.
Now reboot your computer and see if your problem goes away.
If it doesn't then you might need to take a few more steps in order to fix your computer, read these articles carefully, back up your important files before trying any of the suggestions there, and if you need help with some of them come back to the board and ask for help:
http://www.computerhope.com/issues/ch000465.htmhttp://pcsupport.about.com/od/findbyerrormessage/a/ntldrmissingxp.htmPC Edited your message just to make it a bit easier to read
