I think I will start this again from the beginning, just to make sure, since I only see lots of short posts.
Let's start with the BIOS boot menu. According to the user manual you can download here, to access the BIOS:
- Turn on the computer.
- Press the F2 key when the VAIO logo appears.
The BIOS setup screen appears. If not, restart the computer and press the F2 key several times when the VAIO logo appears.
- Press the LEFT ARROW and RIGHT ARROW keys to select "Boot" to display the "Boot" tab.
I suspect you will see something like this (surprisingly, this is not in the manual):
(Image credit: here)
Make sure your options show like in the image above. To change an option, select it with the DOWN ARROW and UP ARROW keys. Press the SPACE BAR key to switch an option from DISABLED to ENABLED and vice-versa. I think you have to press ENTER to change the selections of the boot priorities. You could even move the internal hard disk drive to the second priority.
- Press the LEFT ARROW and RIGHT ARROW keys to select Exit, select Exit Setup, and then press the ENTER key.
- At the confirmation prompt, press the ENTER key.
Your PC should now be configured to boot from the optical drive. Let's go to the next step which is getting a DVD to boot from. I don't know which CD or DVD you are using now. But your best bet is to get an official Windows 7 install DVD ISO image from here. They are useful to repair both Windows Vista and Windows 7 systems. Get the ISO image for Windows 7 Home Premium 32-bit English version (assuming you have a 32-bit system, which was typical while Vista was out).
Once you have downloaded the ISO image you need to burn it to a DVD. You can use both ImgBurn (instructions here) or the Windows 7 USB/DVD Download Tool (instructions here).
Once you have done this, insert the DVD you created in your VAIO first, and then restart your computer. It should display the message "Press any key to boot from CD or DVD...". At this point press any key just once. The "Windows is loading files" message should appear with a white progress bar, then the "Starting Windows" and the Windows logo should appear.
Next you will see this screen, select your keyboard layout and click "Next":
Click on "Repair your computer":
Let the window check for installed operating systems, select your Windows Vista installation and click "Next":
If at this point you see an error message it is because your system is 64-bit and the disk you downloaded is a 32-bit one or vice-versa. Download the right disk and repeat this process. If no operating system appears, then you might have more problems.
Next, click on "Startup Repair":
Let the system check for problems and repair them. Restart and do not press a key when asked to boot again from the CD or DVD, to check if the system works now. If after restarting your PC it still won't boot, repeat this process two or three times more. If it still won't boot, try this manual repair: instead of "Startup Repair" select "Command Prompt".
Here you will find a tutorial to backup your files using this DVD you have now. Follow the tutorial to check the letters of your drives (C:\, D:\ etc.) from Notepad's "Open File" dialog. They may be different in an environment like this: for instance your recovery partition may be labeled "C:", and your Windows Vista may be labeled "D:", although this is rather unusual.
Then mark your Windows Vista partition as active. Use the following commands from the command prompt:
(you are now inside DISKPART>)
select disk 0 (select the hard drive with Windows Vista)
select partition 1 (select the partition with your Windows Vista installation)
You can now try to restart. If it is not fixed, try to manually repair your bootloader. Change directory to E:\boot, where E: is your Windows 7 install DVD (check the drive letter):
cd /d E:\
Run the following command, where C: is your Windows Vista OS partition (check the drive letter):
bootsect.exe /nt60 C:
Then restart your computer.
I hope this comprehensive guide is useful to you. If you have troubles during any step, tell us.
(Images for the recovery environment from here)