How to change the drive letter in a dual boot computer

Chris, that should only be the case if XP is installed last. Then it does indeed steal the boot from Win7, requiring repair of Win7 (mark Active then run Startup Repairs) to add XP using EasyBCD.

No, Vista is modern enough it knows how to configure a Dual Boot, XP doesn't.

I created the DVD and booted from it in EFI mode.Now it is expanding files.I formatted the partition I wanted to install Windows 7.These are the right steps,aren't they?

The drive letter is C: :)
But I can't change the language,so I have to install Ultimate
The original DVD is lost but I have the unbootable(Greek) one.Shall I use this one to install Ultimate or the new Digital River ISO?
The problem is that Anytime Upgrade doesn't accept my key and tells me to type another one

You cannot clean install Anytime Upgrade, it can only be applied to a lower version to unlock a higher version.

If you are following Clean Reinstall Windows 7 as you should be, it shows in Step 1 how to unlock all versions in any ISO so you can choose your licensed version. For which version do you have a Product Key on a COA sticker, either attached to the machine or on a retail copy of Win7?

For example, if you have Home Premium printed on the COA sticker you'd install Home Premium and activate with that Product Key, then insert the Anytime Upgrade key to unlock the higher version. Windows Anytime Upgrade - How to - Windows 7 Forums

I asked you before what exactly you have for the "unbootable (Greek)" installer. Where did you get it? Is this an ISO or is it files? Did you follow the tutorial I provided you earlier to compile files into an ISO, then burn it to disk or write to flash stick?

Thanks for the answers,I managed to install Ultimate:)
It was an error that wasn't actually an error.I want to say that I rebooted the computer and it said Upgrading Windows
How can I change the Recovery Options language?

Do you have a license for Ultimate? Your specs say Professional. You should not install a version for which you do not have a license.

If you Anytime Upgraded to Professional then you need to install the lower version you originally had, activate it, then do the Anytime Upgrade. I just told you that.

You can preserve the Anytime Upgrade with a Win7 backup image described in Clean Reinstall Windows 7 so that you don't have to do this again. The other steps there will assure you get and keep a perfect install just as long as you stick with them.

I have 2 licenses,one for Professional and another for Ultimate.The problem now is RE language.It's in English.How can I change it to Greek?

Activate with Ultimate Product Key, install the Greek Language pack from optional Windows Update.