Sometimes you can corrupt Windows' MBR (Master Boot Record) unintentionally, through installation of another operating system for instance, and you need to repair it before Windows can boot again. The error message that you usually get is "Operating System not found". Under Win 95, 98 and ME, you could have used a standard MS-DOS utility called FDISK in the Command Prompt console. However, FDISK is not distributed with Win NT, 2000, XP, Vista, and 7. So you either have to resort to finding an alternative utility called MbrFix.exe and run it from floppy, optical or USB drive or follow these direction from the installation DVD to repair your MBR. While these directions are specific to Win 7, you maybe able to use the knowledge gained to apply to your particular Win install.

'these instructions' - sounds like you copied only part of an article.

For those that really do need help with this, these instructions work with XP or Vista (Windows 7 is Vista mk II, so it works in Win7, too)

http://helpdeskgeek.com/how-to/fix-mbr-xp-vista/

The Microsoft KB article with the background:

http://support.microsoft.com/kb/927392

When I tried to do this , I got a error message basically telling me I could lose everything on my drive.What I need to do?please help me .

It depends on exactly what you are trying to do - the OP appears to have given some very dangerous options there and you could lose all your data... which option are you trying? FDISK is a very very powerful program that can delete all data on the disk in seconds.