How to change the drive letter in a dual boot computer

Hi,
I would like to know how to have the same letter in the two partitions which I dual boot.For example,when I boot into Windows 8,the boot partition's letter is C:,but Windows 7's partition letter is G:.I shrank the C: partition,formatted the new one and didn't assign any letter.In a previous time,I could have Windows 7 partition as C: when I booted into it and the 8 one had the letter E:.The installation DVD showed the non-assigned-a-letter partition as G:.What shall I do?

If Windows 7 isn't C when it is booted then this is because it was installed from another OS and not correctly booted to install. This blocked the C letter from being available to assign to it.

Windows 7 would not be expected to be C when it is not the booted OS, as the booted OS should always be the only one assigned C.

Windows 7 was installed from inside Windows 8.The G: letter was assigned to the partition I wanted to install 7 without even being assigned by me.I will try a custom installation from Windows 7 and see if the letter is changed to C:

It is never correct to install an OS from another OS, and as I stated it will remove C from being available.

If you know for sure that WIn7 will not boot as C and you want it to do so, then you'll need to boot the Win7 installer to Clean Reinstall Windows 7.

Do you mean that I boot from the DVD?
Anyway,I will try the reinstall right now.
Then I will delete the Windows.old folder,since I backed up the music,pictures,videos and the rest in a USB.

Yes, always boot the installation media, delete the target partition, then recreate it, format and click next to proceed.

I never boot from it,because it gives GPT errors and if I change it to MBR,the Windows 8 partition will be unbootable.The only ISO I have as UEFI bootable is a Digital River ISO which has Ultimate 64 bit.Shall I burn it to a DVD?

You should unlock the ISO with eicfg removal tool first before burning to disk with ImgBurn at 4x speed, choose only your licensed version to install.

If you have a GPT disk then you'll see an EFI System partition in Disk Mgmt. Look now. In that case you must boot the disk as a UEFI device so that you don't have errors. Let us know any other errors so we can help you work through them.

The UEFI Boot settings are as follows:
UEFI Boot:Legacy Support
UEFI Priority:UEFI First
It's in Legacy because if I change it to UEFI it will give 0xc000428 error and the ISO is English of course.I would like to have the option to choose language in Setup

You can choose some Language options on the first screen of installer. Then full Language packs are available in Professional version via optional Windows Update list.

Some BIOS' require Legacy or CSM to install Windows 7 at all. Others you can keep it set to UEFI since you're installing Win7 to UEFI anyway. You'll have to experiment to see, or keep what you have if that's what worked in the past.

Either way, you'll need to turn off Secure Boot in BIOS' Security settings to install Win7.