Hello your windows system is damaged, or the harddrive is failing.
You need to install windows again, to make the pc running again.
The other solution is to use a rescue cd, and try to repair windows
It can be downloaded here:http://www.hiren.info/pages/bootcd
When downloaded burn the iso file to a disc, and let he pc boot from it.
With the programs on the disk you can save some of you files on the harddrive, if its not damaged.
Typical bootmgr missing is caused by virus and trojans, or a failing harddrive.
I can`t give you a guaranty this will work, but a new install of windows will.
Here is the ASUS homepage:http://support.asus.com/download/options.aspx?SLanguage=en
You will need this site for the drivers, for the mainboard.
Just enter the mainboard number.
The files from the site needs to be unziped before you install them.
Hope this will help you