I afraid that it is not possible without OS disk.
You can borrow it form your friend or neighbors and can do. (But of same version 32/64 & Basic/home premium/Ultimate etc).
Follow the procedure below:
- Get your Vista DVD and pop it into your drive.
- Then restart your computer.
- Set your computer to boot from DVD or CD in your BIOS.
Vista will ask you for language and keyboard layout information, after which you will be asked if you want to re-install Vista or repair your existing installation.
After that select Repair. After which Vista's System Recovery Tool will start.
OR
If the previous method does not resolve the problem, you can rebuild the BCD store by using the Bootrec.exe tool in the Windows Recovery Environment. To do this, follow these steps:
- Get your Vista installation DVD and pop it into your drive.
- Then restart your computer.
- Press a key when you are prompted.
- Select a language, a time, a currency, and a keyboard or another input method, and then hit Next
- Click Repair your computer
- Click the operating system that you want to repair, and then click Next
- In the System Recovery Options dialog box, click Command Prompt
- Type bootrec /rebuildbcd and then press "Enter" key.
• If Bootrec runs successfully, it will present you with the installation path of a Windows directory. To add the entry to the BCD store, type Yes . A confirmation message appears that indicates the entry was added successfully.
• If Bootrec cannot find any missing Windows installations, you must remove the BCD store, and then you must re-create it. To do this, type the following commands in the order shown below: (Remember to press ENTER after each command)
bcdedit /export C:\BCD_Backup
ren c:\boot\bcd bcd.old
bootrec /rebuildbcd
- Restart the computer.
This should fix the problem.