I have an asus eeepc 900. i have an error message

you need DVDRW external for install it. And prepare your installer windows too.

hi charmed4031this netbook comes with an internal harddisk on which windows is installed but lacks any optical drives so far.
You will have to get an external CD/DVD-drive plus a Windows-CD for it from which you safely can reinstall windows or replace the missing file. Please find out which windows version you are using(Win200, Win XP Home for e.g.,), lend or buy this external drive, connect devices and let the computer work it's magic, if still facing diffifculties, you know where to find us ;-)