Hmm... XP exists on my single partition. I will open XP, and from there, I will delete the partition (along with XP) and run a command from XP. Can you see how that might not work?
It does not matter what worked on your brother's PC. That is not the PC we are dealing with. It is also possible that you are not aware of some of the detail involved.
Vista will allow an in-place upgrade to Windows 7. Insert DVD, upgrade. No problem. XP does not provide this option., Period.
It is possible that Tews has bailed on this thread. He did not need to assume anything - the failure is self-evident. I will make another, possibly last, effort. Since you do not have a bootable DVD, try to create a bootable USB. USB Windows 7 Installation Key Drive - Create
Your friend at the PC shop likely made you a bootable thumb drive - maybe, maybe not.
In your BIOS, you will need to set the option to boot from USB to allow this to work. If that is not an option, you need a bootable DVD, or a second partition or second hard drive.