If your computer is running windows7,then you can make recovery disc.If windows fail to boot,you can boot your computer using this emergency bootable disc .You can make emergency repair disc by few steps.

1.Go to Start menu >Click on Control Panel
2.Make sure that you selected small icons in view by in control panel
3.Click on Backup and Restore
4.Click on Create a system repair disc in getting window 
5.Now you get a dialogue box.Select the drive you wish to burn
6.Insert a blank disk into that drive.
7.Click on Create disc button and follow the instructions