Hello Petros_K,
I understand you are looking into how to create a System Repair Disc for Windows 7. I am posting an HP Support document: Creating and Using a System Repair Disc (Windows 7) which will provide with the instructions to creating and using an System Repair Disc for Windows 7.
At the command prompt you can use the following commands to repair your Master Boot Record.
bootrec.exe /fixmbr
d:\boot\bootsect.exe /nt60 all /force
del C:\boot\bcd
bootrec.exe /rebuildbcd
I hope I have answered your question to your satisfaction. Thank you for posting on the HP Forums. Have a great day!