help please! trouble installing OS.

[Tmadness]

I tried to install a ssd into my uncle's Acer aspire s5 431 after the original drive failed. The computer originally ran windows 8 and hence was in safe boot mode, as I want to install windows 7 on the ssd I've tried to change the bios mode to legacy however as I proceed to do so by pressing f2 rapidly on boot the computer responds with a beep and the screen says' no os. I've currently removed the cmos battery. What could be the problem?

[Greggers]

So far you say you've changed the drive, moved into legacy mode (why?) and rebooted? Have you actually you know, installed the os?