Boot up problem dell inspiron

Hey.. Mine is a dell inspiron
15(3521),i3
My hard disk has been corrupted.I then changed it to windows 7 from windows 8.It worked properly for some months with that bad disk. But recently it starts up.. Takes longer time to boot... Then suddenly comes with a message that
" Intel UNDI,PXE-2.1(build 083)
copyright (c) 1997-2000 Intel corporation
This product is covered by one or more patents :us5,307.........
PXE-MOF:Exiting PXE ROM
No boot devices found. Press any key to reboot the machine " please help me...
What should I do!!