Disk partitions deleted & Device is write locked

Mac profesionals,

I'm having the following problem;

My cousin would install windows on my macbook air, he putted the usb inside and booted from the usb into windows. deleted all my partitions and couldnt install windows because the GMX partions. Now I want install a new version of osx and after with bootcamp istall windows too. BUT... The device is locked, I can't use the disk utility, I can't restore, I can't reinstall osx. I tried to boot with ''CMD+S'' but the difference I see to my macbook and most internet helping guides is the command line start with ''boot #'' and at my mac ''-sh-3.2#''

If I write the command line ''/sbin/mount -uw /''

I get: 'Device is write locked''
'0 [level 3] [Readuid 0] [Facility com.apple.fs] [Errtype I0] [ErrNo 13] [I0Type Write] [PBlk288552] [LBlk18661]

Please help, I want to use my macbook again and kill my cousin

Update:
When start with ''CMD+s'' It's appear too:
''Root device is mounted read-only''
Singleuser boot -- fsck not done

If someone need more info, please tell..

what Mac?
What version of OS X?

if 10.7 or 10.8, tried booting with command + R ?
if 10.6 or earlier, tried booting from the disc that shipped with your computer?

bobtomay said:

what Mac?
What version of OS X?

if 10.7 or 10.8, tried booting with command + R ?
if 10.6 or earlier, tried booting from the disc that shipped with your computer?

Click to expand...

I'm realy new into mac and just learned littlebit the way around till this happens..
The dvd I got with it, is lost and I allready try to download a new one and put it on usb, but when it reads the usb, a cross apears after 1min.
What version of OSX, I really don't know but the type macbook is an Air A1466 EMC 2559 if this helps.. thnx allready

It would have shipped with 10.7 or 10.8.
It would not have had a disc shipped with it.

Tried booting with cmd + R yet?
Then open Disk Utility after it boots up and repartition the drive.
You'll need to be connected to the internet and it should do an internet recovery install.

From the model number it came with Lion OS X.7 so there were no DVD's. Do as bobtomay suggests and use Lion recovery and proceed from there. Want a good tip? Keep others out of your MBA.

Fingers crossed and keep us posted?

http://support.apple.com/kb/HT4718