Uninstalled Windows 10 and now operating system doesn't work - please help!

Hey guys,

I own an inspiron15z and I installed Windows 10 about a week ago. I didn't like it, so I un-installed to try and go back to Windows 8 but now the entire operating system doesn't work.

A similar issue happened to me last year and I so I have both the operating systems on a CD and thumb drive. Trying to re-boot like I did the last time, but pressing F8 is consistently not working to get to the advanced screen.

All that keeps happening is I am asked to enter my system password twice, the screen goes black and then this message appears:

Intel UNDI, PXE-2.0 (build 083)
Copyright (C) 1997-2000 Intel Corporation

For Atheros PCIE Ethernet Controller v2.1.1.1(12/23/11)

Check cable connection!
PXE-MOF: Exiting Intel PXE ROM

Reboot and Select proper Boot device
or Insert Boot Media in selected Boot device and press a key

So I try inserting both the CD and thumb drive and nothing happens with the thumb drive, it just keeps repeating the message every time I hit a key. The CD, however, takes the screen to the dell logo and then it stays like that. I've left the computer for almost a half hour and the logo never goes away.

Pressing F12, F2 etc all work fine, its just the F8 key that only takes me to the above message.

Any help is greatly appreciated!!