I have a emachine netbook and it say no bootable device

Failed hard drive (assuming one is installed), or could be that it has "lost" the Windows installation.
Possibly a virus, you need to reinstall windows, or hopefully, repair the installation

To do this, as this does not appear to have a DVD rom, you will need the following:

1. A legit copy of windows (whichever is installed currently)
2. A spare computer or preferably an external USB DVD Drive (if the latter, skip to the end)
   
3. The SATA drivers for this laptop (essential if using windows xp, may not need if vista)
   
4. a USB pendrive of at least 4 GB
5. A USB Floppy drive (possibly)
   
6. a copy of win to flash

http://wintoflash.com/home/en/

Uisng the spare computer, extract the contents of the Windows disc to a folder on the spare computers hard drive. Run win to flash and follow the instructions, it will build a boot copy of windows on the pendrive that you specify (it will format the pendrive also).

Copy the sata drivers to the USB floppy drive (you might get away with copying them to the same usb bootable you have just made but I doubt it)

Now, in the bios, set 1st boot to USB drive

Then boot and install, when you reach the F6 prompt, point it to the usb floppy drive and let if find the sata textmode drivers.

Its hould then find your existing windows installation and repair it

IF you ahve a USB DVD / CDROM drive, ignore the above and just install from that, you may need to use a floppy usb to install the sata drivers though

Check the hard disk if working. check as external drive.