Hey Guys!

I got a old Dell 755 SFF.

The computer isn't booting "right". On every start it wont find boot devices. 
But with a press on F12 during boot i can boot from a USB or HDD.

So i pressed F12, installed Windows 7, rebooted and.. yep! nothing.. No boot device found..
USB and HDD are properly recognized by BIOS...
Boot device order in BIOS is: HDD, USB, CD/DVD

May you help me?