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What make and model is your computer? I agree with the others, your computer is trying to boot an image over the network and obviously not finding one available.
You need to check out the boot order set up in your BIOS as mentioned by Aia above. The boot list would normally be set to something like:
Floppy (if you've a really old system! What do you mean, "What's a floppy?")
Hard drive (With no removable media fitted, this is what you should normally boot from.)
Tape (to restore stand alone backups if required)
Network (PXE) (If all else fails, is there an image on the network?)
Note that if your boot order were set up as above and, without any removable media installed, should your hard disk have failed, the BIOS would attempt a PXE boot.
I hope this clarifies rather than clouds the issue.